Preheat the oven to 375°F (190°C). Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool slightly.

In a large skillet over medium heat, add the ground beef and cook until browned, breaking it apart with a spoon. Drain excess fat if necessary.

Add the diced onion and minced garlic to the skillet with the beef. Sauté until the onion is translucent, about 3-4 minutes. Stir in the Italian seasoning, salt, and pepper. Remove from heat.

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

Stir the beef mixture into the cheese mixture until evenly combined.

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

Stuff each cooked pasta shell with the beef and cheese mixture and place them in the baking dish.

Pour the remaining marinara sauce over the stuffed shells and sprinkle the remaining mozzarella cheese on top.

Cover the 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 a few minutes before serving. Garnish with fresh basil or parsley if desired.