Heat a medium size, heavy skillet over a medium heat. Add the ground beef and onion and saute until the meat browns and the onion is translucent, about 5 minutes.Remove from heat and let cool slightly.
Meanwhile, in a medium bowl, mix the ricotta. 1 cup of the mozzarella cheese, 1/4 cup of the Parmesan cheese, the parsley , garlic, salt and pepper. Stir in the meat mixture and set aside.
Brush 1 teaspoon of the olive oil over a large baking sheet. Bring a large pot of salted water to a boil. Working in batches, add the manicotti and cook until softened but still firm to the bite, about 6 minutes. Using a slotted spoon, transfer the manicotti to the baking sheet and let it cool.
Preheat the oven to 350 degrees. Brush the remaining 2 teaspoons of oil over a 13 by 9 by 2-inch glass baking dish. Spoon 1/2 cup of the marinara sauce over the bottom of the prepared dish. Fill the manicotti with the cheese-meat mixture, and arrange the stuffed pasta in a single layerin the prepared dish. Spoon the rest of the sasuce over the manicotti. Sprinkle the remaining cup of mozzarella cheese and then the 1/4 cupp of Parmesasn cheese over the manicotti Dot the top with the pieces of butter
Bake the manicotti until it is heated through and is bubbling from the bottom of the dish, about 35 minutes Let the manicotti stand for 5 minutes and server.
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