Heat a heavy medium skillet over medium heat. Add 1 tsp of the Olive Oil, onion and Sausage. Season with Salt and Pepper. Sauté until the meat browns and the onion is translucent, about 5 minutes. Remove from heat and cool.
Brush 1 tsp of Olive 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 4-6 minutes. Using a slotted spoon, transfer the manicotti from the pot to the oiled baking sheet and cool.
Meanwhile, combine the ricotta, 1 ½ to 2 cups mozzarella cheese, ½ cup parmesan and Parsley. Add the Garlic, Salt and Pepper to taste and mix. Stir the cooled meat mixture into the cheese mixture.
Preheat oven to 350° F.
Brush the remaining 2 tsp of Olive Oil over a 9 x 13 glass baking dish. Spoon 1 ½ cups of 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 it.
Sprinkle the remaining 1 ½ cups of Mozzarella cheese, then the remaining ½ cup of Parmesan Cheese over the stuffed pasta. Dot the entire dish with the Butter pieces. Bake the Manicotti uncovered until heated through and the sauce bubbles on the sides of the dish, about 30-35 minutes. Let the Manicotti stand for 5 minutes and serve.