Bring a large pot of salted water to a boil and cook the jumbo shells until just tender. I always stir gently to keep them from sticking together. Drain them and let them cool slightly so they’re easier to handle.
Heat olive oil in a skillet over medium heat and sauté the onion until soft. Add garlic and ground beef, cooking until the meat is browned and crumbly. Stir in half of the marinara sauce and let it simmer briefly to blend flavors.
In a mixing bowl, combine ricotta, mozzarella, Parmesan, egg, Italian seasoning, salt, and pepper. I like to mix until smooth but not overworked. The filling should be creamy and easy to spoon.
Spread a thin layer of marinara sauce in the baking dish. Fill each shell generously with the cheese mixture and arrange them open-side up. Spoon the meat sauce over the shells so each one is lightly covered.
Cover the dish with foil and bake in a preheated oven at 375°F (190°C) for 25 minutes. Remove the foil and bake another 10 minutes until the cheese is melted and bubbly. Let it rest a few minutes before serving.