Preheat oven to 350°F (175°C).
Cook the pasta shells in boiling water until al dente, according to package instructions. Drain and rinse under cold water. Set aside.
Prepare the beef filling:
Heat olive oil in a large skillet.
Add onion, salt, and pepper; sauté for 4-5 minutes.
Add garlic and cook for 1-2 minutes until fragrant.
Add ground beef; break into chunks and cook until browned.
Stir in Italian seasoning and 1 cup spaghetti sauce. Mix well and remove from heat.
Prepare the cheese mixture:
In a bowl, mix ricotta, mozzarella, Parmesan, and beaten egg.
Add this mixture to the beef filling and combine.
Assemble the dish:
Grease a 9x13-inch baking pan with cooking spray.
Spread the remaining 1 cup spaghetti sauce evenly at the bottom.
Fill each jumbo shell with the beef-cheese mixture and arrange in the pan.
Sprinkle with extra Parmesan and mozzarella cheese.
Bake for 25-30 minutes, until the cheese is melted and bubbly.
Serve hot and enjoy!