Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large bowl, combine the ground turkey, egg, breadcrumbs, diced onion, 2 tablespoons of the Buffalo sauce, garlic powder, and black pepper. Mix gently until just combined.
Divide the mixture among the 8 prepared muffin cups. Press it in lightly without packing it tightly.
Spoon the remaining Buffalo sauce over the tops of the turkey mixture. Bake for 20 to 25 minutes, until the muffins are cooked through and reach 165°F in the center.
While the muffins bake, toss the celery and cabbage with the mayonnaise and apple cider vinegar in a bowl. Chill the slaw until ready to serve.
Let the muffins rest in the pan for 5 minutes, then lift them out carefully. Serve warm with the celery slaw.