Cook the Pasta Shells:

Preheat the oven to 375°F (190°C). Cook the pasta shells in salted boiling water until al dente. Drain and set aside.

Prepare the Filling:

In a skillet, heat olive oil over medium heat. Add the chopped onion and sauté until translucent. Add the minced garlic and cook for another 1 minute. Add the ground beef or sausage, breaking it up with a spoon, and cook until browned. Stir in the chopped spinach and cook until wilted. Season with Italian seasoning, salt, and pepper. Remove from heat.

Mix the Cheeses:

In a large bowl, combine ricotta cheese, half of the mozzarella, and half of the Parmesan. Mix well. Add the cooked meat mixture to the cheese mixture and stir until combined.

Stuff the Shells:

Spread a layer of marinara sauce at the bottom of a baking dish. Fill each pasta shell with the cheese and meat mixture, then arrange them in the baking dish. Pour the remaining marinara sauce over the shells and sprinkle with the remaining mozzarella and Parmesan cheese.

Bake:

Cover the dish with foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.

Serve:

Garnish with fresh basil or parsley before serving.