In a large mixing bowl, combine the ground beef, ground pork, Italian seasoned bread crumbs, onion, garlic, parsley, eggs, Parmesan cheese, and salt and pepper. Using clean hands, mix until everything is evenly combined, being careful not to overwork the meat.
Roll the mixture into meatballs about 1 to 2 inches in diameter. You should get around 20-24 meatballs.
Lightly grease a baking sheet and arrange the meatballs on it in a single layer. Broil on high for about 4–5 minutes per side, turning once, until the meatballs are browned on the outside. They do not need to be fully cooked through at this stage.
Transfer the browned meatballs to the slow cooker.
Pour the spaghetti sauce, crushed tomatoes, and tomato puree over the meatballs. Sprinkle in the crushed red pepper flakes and Italian seasoning. Gently stir or shake the slow cooker so the meatballs are mostly submerged in the sauce.
Cover and cook on LOW for about 4 hours, or until the meatballs are cooked through and very tender. Taste the sauce and adjust with additional salt and pepper if needed.
Serve the meatballs and sauce warm over pasta, or on their own, topped with fresh parsley, basil, and extra grated Parmesan cheese if desired.