Boil the jumbo shells until tender but still firm. Drain and rinse under cold water to stop cooking and prevent sticking. Set aside for stuffing.
In a skillet, heat olive oil and sauté onions and garlic until soft. Add ground beef, Italian seasoning, salt, and pepper, cooking until browned. Stir in ½ cup marinara sauce and cook for 2 more minutes.
In a bowl, mix ricotta, Parmesan, eggs, oregano, and one cup of mozzarella. Combine this mixture with the cooked beef for a creamy, flavorful filling.
Spread 1 cup of marinara sauce in the bottom of a baking dish. Stuff each shell with about 2 tablespoons of filling and arrange in the dish. Top with remaining sauce and mozzarella.
Cover with foil and bake at 375˚F for 40 minutes. Remove foil and bake an additional 5 minutes until golden and bubbly. Garnish with fresh parsley and serve hot.