Preheat your oven to 375°F (190°C).

In a large saucepan over medium heat, add ground beef or Italian sausage and diced onion. Cook until meat is browned (5-7 minutes).

Add minced garlic and sauté for an additional 1-2 minutes.

Stir in crushed tomatoes, tomato paste, oregano, basil, salt, and pepper. Simmer for 10-15 minutes.

In a mixing bowl, combine ricotta, mozzarella, Parmesan, egg, and parsley. Mix well.

Carefully fill each manicotti tube with the cheese mixture and arrange in a greased baking dish.

Pour the meat sauce over the filled manicotti, ensuring they are covered.

Bake in the preheated oven for 25-30 minutes until cheese is bubbling.

Let cool for about 5 minutes before serving.