In a large bowl, combine ground Italian sausage, ground beef, breadcrumbs, 1/4 cup Parmesan cheese, egg, 3 cloves minced garlic, chopped parsley, 1 teaspoon salt, and 1/2 teaspoon black pepper. Mix gently until just combined.
Form the mixture into 1 1/2-inch meatballs and set aside on a plate.
Heat olive oil in a large skillet over medium heat. Add the meatballs in batches and cook until browned on all sides, about 5-6 minutes. Remove meatballs and set aside.
In the same skillet, add chopped onion and cook until softened, about 4 minutes. Add 3 cloves minced garlic and red pepper flakes, cooking for 1 minute until fragrant.
Pour in crushed tomatoes and stir to combine. Bring to a simmer and cook for 5 minutes.
Stir in heavy cream and 1/4 cup grated Parmesan cheese. Season with salt and black pepper to taste.
Return the browned meatballs to the skillet, spoon some sauce over them, cover, and simmer for 15 minutes until meatballs are cooked through.
Garnish with chopped fresh basil leaves before serving.