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 350ºDrizzle 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!