FOR THE COLESLAW (Prepare first): Toss the cabbage and scallions with the mayonnaise, cider vinegar, celery seed & sugar; season with salt and pepper. Allow to macerate & break down in the fridge while you make the meatballs.
FOR THE MEATBALLS: Combine all ingredients, except the BBQ sauce in a large bowl. Line a baking sheet with foil & spray with cooking spray. Form meatballs (about 2 Tbsp each) & place onto prepared baking sheet (makes about 12-14 meatballs.) Bake at 400˚F for 15-16 minutes, or until no longer pink. Heat BBQ sauce in a large saucepan, add baked meatballs & gently fold/toss in the sauce. Remove from heat when it boils/sizzles.
TO ASSEMBLE: Place a saucy meatball on the slider bun and top with fresh coleslaw.