Whisk the sauce: In a medium bowl, whisk together BBQ sauce, grape jelly, ketchup, apple cider vinegar, Worcestershire, Dijon, brown sugar, garlic powder, onion powder, red pepper flakes, salt, and pepper until smooth.
Layer the meatballs: Add the frozen (or chilled, cooked) meatballs to the crockpot in an even layer. If you’re using a small crockpot, it’s okay if they stack slightly.
Pour and toss: Pour the sauce over the meatballs, then gently toss to coat. You want every meatball covered so they cook evenly.
Slow cook: 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 thickened slightly.
Stir and taste: Give everything a gentle stir halfway through.
Adjust seasoning at the end—add a splash more vinegar for tang or a teaspoon of brown sugar for extra sweetness.
Garnish and serve: Top with chopped parsley or green onions. Serve straight from the crockpot for parties, or plate over rice, mashed potatoes, or buttered noodles for dinner.