1. Prepare the Pasta Shells: Cook jumbo pasta shells in a large pot of salted boiling water for 2-3 minutes less than the package directions to ensure they remain firm. Drain and rinse with cold water to stop further cooking.
2. Cook the Ground Beef: Sauté chopped onions and minced garlic in a skillet over medium heat until translucent and fragrant. Add ground beef, breaking it into pieces, and cook until browned thoroughly.
3. Mix the Cheese Filling: In a mixing bowl, combine ricotta cheese, one egg, thawed and squeezed spinach, Italian cheese blend or mozzarella, and Parmesan. Mix until smooth and well-integrated.
4. Assemble the Stuffed Shells: Preheat the oven to 350°F (175°C). Spread half of the beef marinara sauce mixture in a baking dish as the base layer. Stuff each pasta shell with two heaping tablespoons of the cheese mixture. Arrange the stuffed shells in the dish.
5. Add Sauce and Cheese Toppings: Pour the remaining marinara sauce over the stuffed shells to cover them. Generously sprinkle additional mozzarella and Parmesan cheese over the top for a cheesy layer.
6. Bake to Perfection: Cover the baking dish with foil and bake for 40 minutes. Remove the foil and bake for an additional 5 minutes to achieve a golden brown cheese topping.
7. Let Rest and Serve: Allow the dish to rest for a few minutes to let the flavors meld together before serving warm. Serve with optional toppings like freshly chopped parsley for added freshness.