Preheat your oven to 375°F (190°C).

Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside.

In a large skillet, heat the olive oil over medium heat. Add the diced onion and minced garlic, sautéing until the onion is translucent, about 3-4 minutes.

Add the ground beef to the skillet, breaking it apart with a spoon. Cook until browned and fully cooked, about 5-7 minutes. Drain excess fat if necessary.

Stir in the marinara sauce, Italian seasoning, salt, and black pepper. Simmer for 5 minutes, then remove from heat.

In a mixing bowl, combine the ricotta cheese, half of the mozzarella cheese, and half of the Parmesan cheese. Mix well.

Spread a thin layer of the meat sauce on the bottom of a 9x13-inch baking dish.

Stuff each cooked pasta shell with the ricotta mixture and place them in the baking dish, open side up.

Pour the remaining meat sauce over the stuffed shells, then sprinkle the remaining mozzarella and Parmesan cheese on top.

Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.

Let the dish cool for 5 minutes before serving. Garnish with fresh basil if desired.