In a large mixing bowl, combine ground beef, ground pork, Italian seasoned breadcrumbs, grated Parmesan cheese, whole milk, egg, minced garlic, dried oregano, dried basil, crushed red pepper flakes, kosher salt, and black pepper.
Mix all ingredients together gently with your hands until just combined. Do not overmix to keep meatballs tender.
Shape 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, being careful not to overcrowd the pan, and cook until browned on all sides, about 5-7 minutes. Remove browned meatballs and set aside.
In the same skillet, add chopped onion and cook until softened, about 3-4 minutes.
Add minced garlic and cook for an additional 30 seconds until fragrant.
Pour in crushed tomatoes and stir to combine.
Bring the sauce to a simmer, then return the browned meatballs to the skillet.
Cover and simmer on low heat for 15 minutes, or until meatballs are cooked through.
Stir in heavy cream and chopped fresh basil leaves, cooking for 2 more minutes to blend flavors.
Adjust seasoning with salt and pepper if needed.
Serve the meatballs and sauce over cooked pasta or with crusty bread.