Preheat the oven to 425 degrees F. Spray a 9x13-inch casserole dish with cooking spray.

Heat skillet over medium-high heat. Add chopped onion and cook for 1 minute.

Add the ground beef to the onions and continue to cook for 5 minutes. Remove from heat and drain the excess grease before adding it back to the pan.

Add the Worcestershire sauce and continue to cook until ground beef is cooked through.

Turn off the heat and add the golden mushroom soup, ketchup, mustard, relish, and 4 ounces of cheddar cheese to the ground beef and stir to combine.

Transfer the ground beef to the prepared casserole dish and top with the remaining 4 ounces of cheddar cheese.

Layer the tater tots on top of the cheese, covering the entire top of the casserole. Lightly spray the tater tots with cooking spray.

Bake in the preheated oven for 30 minutes.

While the casserole bakes make the sauce. In a small bowl whisk together all of the ingredients until well combined.

When it has finished baking the cheese is melted, and the tater tots are crisp, remove from the oven and let sit for 10 minutes. Then cut and top each portion with Big Mac sauce before serving.