Choose your cooking method. For true dump-and-go ease, use a slow cooker. You can also use a large pot or Dutch oven on the stovetop.
Add the meatballs. Place frozen meatballs straight into the slow cooker or pot. No need to thaw.
Mix the sauce. In a bowl, stir together barbecue sauce, grape jelly, brown sugar (if using), apple cider vinegar, Worcestershire, garlic powder, onion powder, and red pepper flakes.
The sauce should be smooth and glossy.
Combine. Pour the sauce over the meatballs and toss to coat evenly. If using a pot, stir gently to avoid breaking them apart.
Cook. Slow cooker: Cook on Low for 4–6 hours or on High for 2–3 hours, until heated through and saucy.
Stovetop: Simmer on low heat for 20–30 minutes, stirring occasionally to prevent sticking.
Oven: Combine in a baking dish, cover with foil, and bake at 350°F (175°C) for 35–45 minutes, stirring once.
Finish and serve. Once the sauce is thick and clings to the meatballs, garnish with chopped parsley or green onions. Serve hot.