Using a paring knife, cut 3 slits in the meatiest portion of each drumstick.
Place buttermilk, onion, garlic, and drumsticks in a large zip-top plastic bag; refrigerate 1 hour.
Preheat oven to 42
Place a rack on a jelly-roll pan; coat the rack with cooking spray.
Remove the drumsticks from the bag; discard the marinade.
Sprinkle the drumsticks with pepper and salt.
Combine panko, cheese, and chives in a shallow dish. Dredge each drumstick in panko mixture. Arrange drumsticks in a single layer on the rack; coat drumsticks lightly with cooking spray.
Bake drumsticks at 425 for 30 minutes or until chicken is done.
Combine mayonnaise and remaining ingredients in a small bowl.