Tennessee Onion Casserole (Cheesy Southern Side Dish)
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This Tennessee onion casserole turns a few pounds of sweet onions into a buttery, cheesy side dish with cheddar, Gruyère, and Parmesan. A little smoked paprika, fresh thyme, and cayenne balance the sweetness of the onions, making this an easy addition to Thanksgiving, Christmas, or Sunday dinner.
My Southern roots show up in so many of the recipes we share, especially when it comes to holiday side dishes. Give me a table full of casseroles, dressing, and vegetables cooked with plenty of flavor, and I’m happy! These cheesy baked onions fit right in with our Southern Thanksgiving side dishes when you want something a little different to serve alongside the classics.
The onions go into the dish raw, so there’s no skillet to stand over before baking. Covering the casserole first helps them soften, while the uncovered baking time gives the cheese a chance to turn golden in spots.
Cheddar adds familiar cheesy flavor, Gruyère brings a nutty richness, and Parmesan finishes things off with a savory topping. Layering the cheese between the onions means you get more than just a cheesy bite off the top.
What Are Tennessee Onions?
Tennessee onions are a Southern-style side dish made by baking sliced onions with butter, seasonings, and cheese. The onions are the main ingredient here, turning soft and sweet as they cook.
Our version uses thinly sliced sweet onions and three cheeses, along with a little brown sugar, smoky seasoning, and fresh thyme. There’s no canned soup, cream sauce, or breadcrumb topping.
Ingredients You’ll Need
You’ll find the complete measurements in the recipe card below.
- Sweet onions: Vidalia onions are a great choice, but you can use another variety labeled sweet onions. Their mellow flavor works especially well with the cheese and seasonings.
- Olive oil: Helps coat the onion slices so the seasoning sticks throughout the casserole.
- Salt and black pepper: Season the onions before they’re layered into the dish.
- Brown sugar: Adds a little sweetness to balance the savory cheese and spices.
- Smoked paprika: Brings smoky flavor without needing smoked cheese.
- Garlic powder and mustard powder: Add savory flavor throughout the onions. Use dry mustard powder, not prepared mustard.
- Fresh thyme: Finely chop it so you get a little in each bite.
- Cayenne pepper: Gives the casserole some heat to go with the sweet onions.
- Cheddar cheese: Use mild for a gentler flavor or sharp for a more pronounced cheddar taste.
- Gruyère cheese: Adds nutty flavor and melts into the onion layers.
- Parmesan cheese: Goes over the top for a salty, savory finish.
- Butter: Cut it into small pieces so it melts across the casserole as it bakes.
How to Make Tennessee Onion Casserole
Step 1 – Prepare the oven and dish. Preheat the oven to 350 degrees F. Spray a 9×13-inch baking dish with nonstick cooking spray.
Step 2 – Slice the onions. Peel and thinly slice the sweet onions, preferably with a mandoline for thin, even slices. Separate any slices that are stuck together so they can be coated with seasoning.
Step 3 – Coat with oil. Place the onion slices in a large bowl, drizzle with the olive oil, and toss until the slices are lightly coated.
Step 4 – Mix the seasoning. In a small bowl, combine the salt, brown sugar, black pepper, smoked paprika, garlic powder, mustard powder, fresh thyme, and cayenne. Break up any lumps of brown sugar.
Step 5 – Season the onions. Sprinkle the seasoning mixture over the onions and toss well. Lift and turn the slices as you mix so the seasoning reaches the onions at the bottom of the bowl.
Step 6 – Add the first layer. Spread half of the onions in the prepared baking dish. Sprinkle with 2 ounces of shredded cheddar and 2 ounces of shredded Gruyère.
Step 7 – Finish layering. Add the remaining onions, then top with the remaining cheddar and Gruyère. Sprinkle the grated Parmesan over the top.
Step 8 – Add the butter. Cut the butter into small pieces and dot them evenly over the casserole.
Step 9 – Bake covered. Cover the dish tightly with foil and bake for 45 minutes. The onions will begin to soften and settle as they release their juices.
Step 10 – Uncover and finish baking. Carefully remove the foil and continue baking for 30 minutes. The cheese should be melted and golden in spots, and the onions should be tender when pierced with a fork. Remove from the oven and serve hot.
Expert Tips & Tricks
- Keep the slices even. Thick pieces can stay firm while thinner ones soften. If using a knife, focus on consistent, thin slices.
- Shred the cheese from a block when possible. It generally melts more smoothly than cheese coated with anti-caking ingredients.
- Check the onions in the center. If they still feel firm at the end of baking, cover the dish again and continue cooking until tender. This protects the cheese while the onions finish softening.
What to Serve with Tennessee Onion Casserole
For Thanksgiving, serve these onions with roasted turkey, Southern cornbread dressing, and creamed corn casserole. The savory cheese and smoky seasoning give you another flavor to enjoy alongside those familiar holiday favorites.
Planning Christmas dinner? Pair the onions with ham and sweet potato casserole with marshmallows. You’ll have a little sweet, a little savory, and plenty of comfort food on the table.
They’re also right at home with pork chops, roast chicken, or steak and a scoop of creamy mashed potatoes. Leftover cheesy onions make a tasty topping for a burger or roast beef sandwich, too!
Make Ahead Instructions
To get a head start, slice the onions and shred the cheeses up to a day ahead. Refrigerate them in separate airtight containers. Mix the dry seasonings separately, then add the fresh thyme when you’re ready to assemble.
Wait to toss the onions with the salt and other seasonings until you’re ready to bake. Salt draws moisture from the onions, so prepping the ingredients separately helps keep extra liquid from collecting before the casserole goes into the oven.
Storage and Reheating
Transfer leftovers to shallow airtight containers and refrigerate within 2 hours. Store in the refrigerator for 3–4 days.
Reheat a larger portion in a covered oven-safe dish at 350 degrees F until the center reaches 165 degrees F. Individual servings can be reheated in the microwave in short intervals, stirring between intervals when possible for even heating. These storage times and reheating temperatures follow USDA leftover guidance.
For the best texture, enjoy the casserole fresh or refrigerated. Freezing and thawing can leave the onions softer and release extra moisture.
Frequently Asked Questions
Yes, but the flavor will be stronger and less mellow. Sweet onions are the best choice for the balance of sweetness, cheese, and spice in this recipe.
Swiss cheese is a good option for a similar nutty flavor. Gouda also melts well, though it will give the casserole a different flavor. Use the same weight as the Gruyère.
The cayenne gives this version some heat. If you prefer a milder casserole, reduce or leave out the cayenne. You’ll still get smoky flavor from the paprika.
Onions naturally release moisture as they cook, and this recipe doesn’t contain flour or another thickener. Some buttery onion juices are expected. The uncovered baking stage lets some moisture evaporate, but the finished dish should be juicy rather than dry.
Yes. Divide the doubled recipe between two 9×13-inch dishes rather than piling it into one deeper dish. Leave room for air to circulate around both pans and check each casserole for tender onions before serving.
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Tennessee Onion Casserole
Ingredients
- 3 pounds Sweet onions
- 2 tablespoons Olive oil
- 2 teaspoons Salt
- 2 teaspoon Brown sugar
- 1 teaspoon Black pepper
- 1 teaspoon Smoked paprika
- 1 teaspoon Garlic powder
- 1 teaspoon Mustard powder
- 1 teaspoon Fresh thyme, finely chopped
- ½ teaspoon Cayenne pepper
- 4 ounces Shredded cheddar cheese
- 4 ounces Shredded Gruyere
- ½ cup Parmesan Cheese
- 3 tablespoons Butter
Instructions
- Preheat oven to 350 degrees F. Spray a 9×13 inch baking dish with non-stick spray.
- Thinly slice the onions, preferably with a mandoline to get the thinnest slices.
- Add the onion slices to a large bowl and drizzle the olive oil on top. Toss the onion slices in the olive oil.
- In a small bowl combine the salt, brown sugar, black pepper, smoked paprika, garlic powder, mustard powder, thyme, and cayenne pepper. Mix well.
- Sprinkle the seasoning over the onions and toss until well coated.
- Spread half of the onion slices in a 9×13 inch baking dish and top with 2 ounces of shredded cheddar and 2 ounces of shredded Gruyere.
- Add the remaining onion slices and top with remaining cheddar and Gruyere cheese.
- Top with the grated Parmesan cheese.
- Cut the 3 tablespoons of butter into small pieces and dot it over the top of the casserole.
- Cover the dish tightly with foil and place in the oven to cook for 45 minutes.
- After 45 minutes carefully remove the foil from the dish and continue to bake for 30 minutes more.
- Remove from oven and serve hot.
Nutrition
