Preheat oven to 425 F.
To make the marinade:
In a gallon-sized sealable plastic bag or container, combine the buttermilk, mustard, garlic and hot sauce. Mix until well blended.
Add chicken, seal the bag and turn to coat.
Let chicken marinate in the refrigerator for at least 30 minutes or up to 8 hours.
To make the chicken:
Line a baking sheet with foil. Set a wire rack on the baking sheet and coat it with cooking spray.
In a gallon-sized sealable plastic bag, place flour, ground flaxseed, paprika, thyme, baking powder, sage and pepper and mix until well blended.
Remove chicken legs from marinade. Place 1 or 2 pieces of chicken at a time in the bag containing the flour mixture and shake to coat.
Shake off excess flour and place the chicken on the prepared rack. Throw away any leftover flour mixture and marinade.
Spray the chicken pieces with cooking spray.
Bake the chicken until it is golden brown and the internal temperature is 165 F, about 40-50 minutes.