Prepare ground beef mixture. Heat skillet over medium heat with a drizzle of olive oil. Add onion, garlic and ground beef and season with a little salt and pepper. Cook, breaking up ground beef, until onion is translucent and beef is crumbled and browned and cooked through, about 7-8 minutes. Set aside to cool.
Cook manicotti. Drizzle a little olive oil all over a large baking sheet to prevent sticking. Bring a large pot of salted water to boil and cook pasta tubes until softened but still firm, about 6 minutes. Remove to oiled baking sheet to cool.
Prepare filling. Combine ricotta, 2 cups of shredded mozzarella cheese, ½ cup grated Parmesan and parsley. Add salt and pepper to taste and mix well. Stir the cooled meat mixture into the cheese mixture and combine well.
Preheat oven to 350º.
Drizzle a couple more teaspoons of olive oil into bottom of 9" by 13" glass baking dish. Spread 1½ cups marinara over bottom of prepared dish. Fill the manicotti tubes with the cheese-meat mixture and arrange the stuffed pasta in one layer in the prepared dish. Spoon the remaining 1½ cups marinara over the top.
Sprinkle with the remaining 1 cup shredded mozzarella, then the remaining ½ cup of Parmesan and then dot the entire dish with the butter pieces. Bake the manicotti uncovered until heated through and bubbly, about 35 minutes. Let dish rest for 5 minutes or so and serve. Enjoy!