Prepare the Sauce:
Heat olive oil in a large saucepan over medium heat. Add onions and sauté until translucent, about 3–4 minutes. Stir in garlic and cook 1 more minute.
Add crushed tomatoes, basil, oregano, salt, and pepper. Simmer for 15–20 minutes, stirring occasionally.
Make the Cheese Filling:
In a medium bowl, combine ricotta, mozzarella, Parmesan, egg, parsley, salt, and pepper. Mix until smooth and creamy.
Stuff the Manicotti:
Carefully fill each cooked manicotti shell with the cheese mixture using a spoon or piping bag.
Assemble the Dish:
Preheat oven to 350°F (175°C). Spread a thin layer of sauce in the bottom of a 9×13 inch baking dish. Arrange stuffed manicotti in a single layer. Pour remaining sauce over the top and sprinkle with extra mozzarella and Parmesan, if desired.
Bake:
Cover with foil and bake for 25 minutes. Remove foil and bake an additional 10 minutes, until the cheese is melted and bubbly.
Serve:
Garnish with fresh parsley and serve hot.
Serving and Storage Tips:
