In a large pot of boiling salted water, cook the manicotti shells for 4–6 minutes, allowing them to soften slightly but remain firm. Drain the shells carefully and lay them on a cooling rack to prevent sticking while you prepare the filling.

In a skillet over medium heat, add 2 tablespoons of olive oil, and sauté the coarsely chopped onion until it becomes translucent, about 3–5 minutes. Next, add the ground beef to the skillet, breaking it up with a spoon. Season with salt, pepper, and minced garlic, browning the meat until fully cooked, which should take around 6–8 minutes.

In a large mixing bowl, combine the sautéed beef mixture with 15 ounces of ricotta cheese, 2 cups of shredded mozzarella, ¾ cup of grated Parmesan, and chopped fresh parsley. Mix everything well until the filling is creamy and evenly incorporated.

Using a spoon or a piping bag for precision, gently fill each manicotti shell with the cheese and beef mixture.

Spread a layer of marinara sauce evenly across the bottom of a 9x13-inch baking dish. Place the stuffed manicotti shells in a single layer over the sauce, then pour the remaining marinara sauce on top. Sprinkle the remaining mozzarella and Parmesan cheese over the entire dish and dot with small pieces of butter.

Preheat your oven to 350°F (175°C). Cover the baking dish with aluminum foil and bake for 30–35 minutes, until the sauce is bubbly and the cheese on top is golden and slightly crispy.

Allow the stuffed manicotti to cool for a few minutes before serving.