Preheat oven to 350° F. Cook shells according to package directions and add olive oil to the water. (Prevents sticking) Cook and crumble sausage in a large skillet until browned then set aside to drain on a paper towel-lined plate.
In a large bowl, add ricotta, cream cheese, Parmesan, 1 cup mozzarella, spinach, onion, garlic, egg, parsley, Italian seasoning, salt, pepper, and sausage. Stir to combine well.
Lightly grease one 9x13 baking dish and one 8x8 (or 11x7) baking dish with cooking spray. Add enough marinara to coat the bottom of each pan lightly. Stuff each cooked shell with about 2 tablespoons filling and place in the pan close together, open side up. Cover shells with remaining marinara.
Cover with foil and bake for 35-40 minutes. Uncover and add remaining 1 cup mozzarella. Place back in the oven and bake for 5-10 more minutes.
Notes
These shells freeze very well for up to three months. Let thaw overnight in the refrigerator before baking.