BBQ Chicken Sliders

1 14-oz. bag prepared slaw mix (purple cabbage, green cabbage, and carrots)

1/4 cup chopped fresh flat-leaf parsley

1/4 cup mayonnaise

2 tablespoons apple-cider vinegar

2 teaspoons Dijon mustard

1 teaspoon celery seeds

1/4 teaspoon kosher salt

1/4 teaspoon black pepper

1 pound ground chicken

1/4 cup barbecue sauce, plus more for serving

1/4 cup finely chopped red onion

1/4 teaspoon kosher salt

Nonstick cooking spray

8 slider buns