Make the sauce base. In a medium bowl, whisk together cranberry sauce, chili sauce, brown sugar, vinegar, Worcestershire, Dijon, garlic powder, onion powder, and red pepper flakes. Taste and adjust sweetness, tang, or heat.
Add meatballs to the slow cooker. Place the frozen meatballs in a 5- to 6-quart slow cooker. No need to thaw.
Pour and toss. Pour the sauce over the meatballs and gently stir to coat them evenly.
Cook low and slow. Cover and cook on Low for 4–6 hours or on High for 2–3 hours, until the meatballs are heated through and the sauce is glossy.
Stir and season. Give everything a gentle stir.
Add a pinch of salt and pepper if needed.
Garnish and serve. Sprinkle with chopped parsley or chives. Serve straight from the slow cooker with toothpicks or over rice, noodles, or mashed potatoes.