In a large bowl, combine the breadcrumbs and milk. Let sit for 5 minutes until the breadcrumbs absorb the milk.
Add the egg, ground Italian sausage, ground beef, grated Parmesan cheese, minced garlic, chopped parsley, 1 teaspoon salt, and 1/2 teaspoon black pepper to the breadcrumb mixture. Mix gently until just combined.
Form the mixture into 1 1/2-inch meatballs and place them on a baking sheet.
Heat olive oil in a large skillet over medium heat. Add the meatballs in batches and cook until browned on all sides, about 5 minutes per batch. Remove meatballs and set aside.
In the same skillet, add the chopped onion and cook until softened, about 5 minutes.
Add the minced garlic and red pepper flakes and cook for 1 minute until fragrant.
Pour in the crushed tomatoes, stir to combine, and bring the sauce to a simmer.
Return the meatballs to the skillet, cover, and simmer gently for 15 minutes until the meatballs are cooked through.
Stir in the heavy cream and chopped fresh basil. Simmer for an additional 5 minutes.
Season the sauce with salt and black pepper to taste.
Serve the meatballs hot over cooked pasta or with crusty bread.