Make or prep the meatballs. If using frozen, there’s no need to thaw; they can go straight into the slow cooker. For homemade, roll 1–1.5-inch balls and brown them lightly in a skillet for extra flavor, or bake at 400°F (200°C) for 12–15 minutes until just set.
Mix the sauce. In a bowl, whisk together the BBQ sauce, smoked paprika, Worcestershire, apple cider vinegar, maple syrup or brown sugar, tomato paste, garlic, onion powder, chili or chipotle powder, broth, and a pinch of salt and pepper.
Load the slow cooker. Add the meatballs to a 4–6 quart slow cooker. Pour the sauce over the top and toss gently to coat everything.
Cook low and slow. Cover and cook on Low for 4–6 hours or High for 2–3 hours until the meatballs are heated through and the sauce is glossy and clings to the meat.
Adjust and finish. Taste the sauce.
Add more vinegar for brightness, a pinch of sugar for sweetness, or a splash of broth if it’s too thick. Keep warm on the slow cooker’s warm setting until serving.
Serve. Garnish with chopped parsley or green onions. Serve with toothpicks as an appetizer, or spoon over mashed potatoes, rice, buttered noodles, or toasted rolls.