In a large, deep saucepan melt butter and olive oil over medium-high heat. Add the onions and cook until softened and translucent, about 5 minutes, stirring often. Add the garlic and continue to cook for one minute. Add the tomatoes, marinara sauce, Italian seasoning, sugar, salt and pepper. Bring to a boil, then lower the heat to a simmer and cook for 5 minutes. Add the heavy cream, torn basil leaves, shredded mozzarella, Parmesan and meatballs and stir until well combined and cheese is melted. Add the drained, cooked pasta and stir until well combined. Serve immediately with extra Parmesan cheese, if desired.