Rinse chicken breasts and pat dry. Cut into serving-size pieces.
Combine buttermilk and mustard in a bowl. Add chicken pieces and turn to coat well. Let marinate for about 10 minutes.
Preheat oven to 375 degrees F.
Place pecans on a plate or shallow bowl. Remove chicken pieces from buttermilk mixture and shake off excess. Dredge on all sides in pecans and place pieces on baking sheet. Season with seasoning salt and pepper.
Bake about 30 minutes, or until browned and chicken tests done.