Make the sauce base. In a large pot or saucepan, whisk together grape jelly, chili sauce, ketchup, apple cider vinegar, Worcestershire, garlic powder, onion powder, and red pepper flakes. Warm over medium heat, stirring until smooth and glossy, about 3–4 minutes.
Taste and adjust. Sauce should be sweet with a bright tang.
Add a pinch of salt and black pepper. If it’s too sweet, stir in another teaspoon of vinegar. If you want more kick, add a dash more red pepper flakes.
Add the meatballs. Stir in frozen meatballs, coating them well with sauce.
Cover and simmer on low for 15–20 minutes, stirring occasionally, until heated through and the sauce thickens slightly.
Reduce to glaze. Remove the lid and let the sauce bubble gently for 5 minutes more, stirring now and then, until it clings to the meatballs. Don’t boil hard—gentle heat keeps the sauce silky.
Garnish and serve. Sprinkle with chopped parsley or chives. Serve straight from the pot with toothpicks, or spoon over rice, mashed potatoes, or into slider buns.
Slow cooker method. Add all sauce ingredients to a slow cooker and whisk.
Stir in frozen meatballs. Cook on High for 2–3 hours or on Low for 4–6 hours, stirring once or twice. Switch to Warm for serving.
Oven method. Place frozen meatballs in a baking dish.
Whisk sauce separately and pour over. Cover with foil and bake at 350°F (175°C) for 30 minutes, then uncover and bake 10–15 minutes more until bubbling.