In a large mixing bowl, combine the ground beef, ground pork (if using), breadcrumbs, grated Parmesan cheese, chopped parsley, egg, minced garlic, onion powder, salt, black pepper, and crushed red pepper flakes. Using your hands or a spoon, mix until well combined and uniform in texture.
Shape the mixture into bite-sized meatballs, roughly 1 inch in diameter. Place each formed meatball on a baking sheet lined with parchment paper for easy transfer.
Preheat your slow cooker by setting it to low.
Heat a non-stick skillet over medium heat. In batches, add the meatballs and brown them on all sides, cooking for about 5-7 minutes. This browning step adds depth of flavor, but if you desire a quicker option, you can skip this step and place the raw meatballs directly into the slow cooker.
In a mixing bowl, combine the marinara sauce and BBQ sauce, stirring until thoroughly blended.
Carefully transfer the browned (or raw) meatballs into the slow cooker, coating them well with the sauce mixture.
Cover the slow cooker and let cook on low for 6-8 hours or on high for 3-4 hours, until the meatballs are thoroughly cooked and tender.
When ready to serve, gently stir the meatballs to ensure they are well coated in sauce before plating.
Notes
Serve with toothpicks for easy serving and garnish with fresh basil.