Preheat the oven to 350°F (175°C). Boil salted water in a large pot and cook jumbo pasta shells until al dente, about 8-10 minutes. Drain and rinse under cold water.

In a skillet, heat olive oil over medium heat. Sauté chopped onion until translucent, add garlic and ground beef, cooking until browned. Stir in Italian seasoning and spaghetti sauce.

In a bowl, mix ricotta, shredded mozzarella, grated parmesan, and egg until creamy. Fold in cooled beef mixture to combine.

Grease a baking dish and spread spaghetti sauce on the bottom. Fill the shells with cheese and beef mixture, placing seam side up.

Top with mozzarella and parmesan cheese, cover with foil, and bake for 25-30 minutes. Remove foil in the last 5-10 minutes to brown the cheese.

Let the dish cool for 5 minutes before serving. Garnish with parsley or basil as desired.