Whisk buttermilk, oil, hot pepper sauce, mustard, garlic, 1 teaspoon salt and 1/2 teaspoon pepper in large bowl to blend well.
Add onion, then chicken and turn to coat. Cover; chill at least 3 hours or up to 1 day, turning chicken occasionally. *NOTE: I prefer letting the marinade process go for one day and I put the prepared marinade and chicken into a large zip bag. It makes it easier to turn the chicken.
Place racks on 2 large rimmed baking sheets. **NOTE 2: Line the baking sheets with aluminum foil to make your life easier. And spray the racks with PAM, as the chicken tends to stick to the rack a bit. Not bad, but it does.
Whisk breadcrumbs, cheese, flour, thyme, paprika, cayenne and 1 teaspoon salt in large baking dish (I use a large zip bag) to blend.
Remove chicken from marinade, allowing excess to drip off.
Add chicken to breadcrumb mixture and turn to coat completely.
Arrange chicken, skin side up, on racks on baking sheets. **Let stand 30 minutes.
Preheat oven to 425°F.
Drizzle butter over chicken. Bake until crisp, golden and cooked through, about 50 minutes.