Prepare the Sauce Base: In a medium mixing bowl, whisk together the BBQ sauce, bourbon, ketchup, brown sugar, Worcestershire sauce, minced garlic, and diced onion until completely smooth.
Load the Slow Cooker: Place the frozen meatballs into the bottom of your 6-quart slow cooker. Ensure they are distributed evenly.
Pour the Glaze: Pour the sauce mixture over the frozen meatballs, making sure every piece is coated.
Stir Gently: Using a large wooden spoon, carefully toss the meatballs until the sauce covers all of them.
Slow Cook: Cover the slow cooker and cook on LOW for 3 to 4 hours.
Thicken: For a thicker glaze, remove the lid during the final 30 minutes of cooking.
Serve: Garnish with fresh chopped parsley or green onions and serve warm.