Place chicken in a zip-top plastic bag. Combine buttermilk, egg, 1/4 teaspoon onion powder, and 1/4 teaspoon red pepper; add to bag. Seal. Marinate in refrigerator 4 hours.
Preheat oven to 425 degrees. Remove chicken from bag; discard marinade. Sprinkle kosher salt over chicken. Combine 1/2 teaspoon onion powder, 1/2 teaspoon red pepper, flour, black pepper, and celery salt in a shallow dish. Dredge chicken in flour. Place chicken on a wire rack.
Heat a large ovenproof skillet over medium-high heat. Add oil. Add chicken; sauté 4 minutes. Turn chicken over. Bake chicken at 425° for 10 minutes.