Begin by bringing a large pot of salted water to a boil. Add the manicotti shells and cook them for 4-6 minutes until slightly softened. Drain and set aside to cool.

In a large skillet, heat olive oil over medium heat. Add the chopped onion and sauté for 3-5 minutes until translucent and fragrant.

Add the ground beef to the skillet and cook, breaking it apart, until browned, about 5-7 minutes. Season with salt, pepper, and garlic for the last minute of cooking.

In a mixing bowl, combine the beef mixture with ricotta, mozzarella, parmesan, and parsley. Mix until well blended.

Use a spoon or piping bag to stuff each manicotti shell with the filling, packing tightly but not overstuffing.

Spread marinara sauce on the bottom of a baking dish. Place the stuffed manicotti in a single layer and pour remaining sauce over the top. Sprinkle with mozzarella and parmesan.

Preheat oven to 350°F (175°C). Cover the dish with foil and bake for 30-35 minutes until the sauce is bubbling and cheese is golden.

Allow the manicotti to cool for 5-10 minutes before serving. Garnish with parsley if desired.