Preheat oven to 375 degrees F.
Cook jumbo shells according to the instructions on the back of the box. Once cooked drain and run under cold water to stop the cooking. Let the pasta sit in strainer to cool while you ready the rest of the ingredients.
Place the baby spinach in a microwave safe bowl with 3 tablespoons of water. Wet a paper towel and place the wet paper towel on top of the spinach. Microwave for 2-3 minutes, until fully cooked.
Pour the cooked spinach into a strainer and let the water drain out until it cool completely. Then give it a few good squeezes to get as much excess water out as possible.
Finely chop the spinach up and set aside.
In a large bowl combine the ricotta, 2 ounces of shredded mozzarella, Parmesan cheese, Italian seasoning, onion powder, garlic powder, salt, pepper, cooked spinach,and egg. Stir together until well mixed.
Fill shells with approximately 2 tablespoons of filling each.
Pour 1 cup of sauce in the bottom of a 9x13 inch casserole dish.
Add stuffed shells to the dish and then top with remaining sauce.
Sprinkle with remaining 3 ounces of mozzarella on top. Feel free to add more if you like!
Bake for 25 - 35 minutes or until warmed through.