Heat a heavy medium skillet over medium heat. Add 1 teaspoon of the olive oil, onion and ground beef. Season with salt and pepper. Saute until the meat browns and the onion is translucent, about 5 minutes. Remove from the heat, and cool.
Brush 1 teaspoon of oil over a large baking sheet. Cook the manicotti in a large pot of boiling salted water until slightly softened, but still very firm to the bite, about 4 to 6 minutes. Using a slotted spoon, transfer the manicotti from the pot to the oiled baking sheet and cool.
Meanwhile, combine the ricotta, 1 1/2 to 2 cups mozzarella cheese, 1/2 cup Parmesan, and parsley. Add the garlic, salt, and pepper to taste, and mix. Stir the cooled meat mixture into the cheese mixture.
Preheat the oven to 350 degrees F.
Brush the remaining 2 teaspoons of oil over a 13 by 9 by 2-inch glass baking dish. Spoon 1 1/2 cups of the marinara sauce over the bottom of the prepared dish. Fill the manicotti with the cheese-meat mixture. Arrange the stuffed pasta in a single layer in the prepared dish and spoon the remaining sauce over.
Sprinkle the remaining 1 1/2 cups of mozzarella cheese, then the remaining 1/2 cup of Parmesan over the stuffed pasta. Dot entire dish with the butter pieces. Bake the manicotti uncovered until heated through and the sauce bubbles on the sides of the dish, about 30 to 35 minutes. Let the manicotti stand 5 minutes and serve.
Recipe courtesy of Giada De Laurentiis