Make the sauce: In a bowl, whisk together BBQ sauce, grape jelly, honey, mustard, vinegar, Worcestershire, garlic powder, onion powder, smoked paprika, and red pepper flakes (if using).
Prep the slow cooker: Lightly grease the crockpot insert or use a liner for easy cleanup.
Add meatballs: Place frozen or chilled cooked meatballs into the crockpot.
Pour the sauce over the top and toss to coat.
Cook low and slow: Cover and cook on Low for 3–4 hours or on High for 1.5–2 hours. Stir once or twice so the sauce warms evenly.
Finish and serve: Once hot and glossy, switch to Warm. Garnish with chopped parsley or chives.
Serve with toothpicks for appetizers or over rice, mashed potatoes, or buttered noodles for dinner.
Optional homemade meatballs: Mix the ground meat with breadcrumbs, eggs, onion, milk, salt, pepper, and garlic powder. Roll into 1–1.5 inch balls. Brown in a skillet for 5–7 minutes to set the shape, then add to the crockpot with the sauce and cook as directed.