Preheat the oven to 350°F (175°C).

In a large pot, fill it with water, add a pinch of salt, and bring to a boil. Cook the pasta shells according to package instructions until al dente. Drain and set aside.

In a large skillet, heat olive oil over medium heat. Add chopped onions and minced garlic, sautéing until translucent. Add ground beef and cook until browned. Season with salt, pepper, and Italian seasoning.

In a bowl, combine ricotta cheese, half of the mozzarella cheese, and parmesan cheese. Mix well and add the cooked beef mixture. Stir until thoroughly combined.

Carefully fill each cooked shell with the cheese and beef mixture.

In a 9x13 inch baking dish, spread a layer of marinara sauce on the bottom.

Place the stuffed shells in the dish and top with the remaining marinara sauce. Sprinkle the remaining mozzarella cheese over the top.

Cover the dish with aluminum foil and bake for 20 minutes.

Remove the foil and bake for an additional 10 minutes, or until the top is bubbly and golden.

Allow the dish to cool slightly before serving.