Cook the manicotti shells in a large pot of salted water until slightly under al dente, about 6-7 minutes. Drain and cool on a kitchen towel.

In a skillet, brown ground beef with diced onion and minced garlic over medium heat for 8-10 minutes. Drain excess fat and stir in crushed tomatoes, basil, and oregano. Simmer for 15-20 minutes.

In a mixing bowl, combine ricotta, mozzarella, Parmesan, and beaten egg. Mix well and season to taste with salt and pepper.

Fill each manicotti shell with the cheese mixture using a piping bag or spoon.

Preheat oven to 375°F (190°C). Spread meat sauce in a baking dish and arrange the stuffed shells in a single layer. Pour remaining sauce over the shells.

Sprinkle mozzarella on top, cover with foil, and bake for 30 minutes. Remove foil for the last 10-15 minutes to brown the cheese.

Let the dish rest for about 10 minutes before serving.