In a large mixing bowl, mix together the buttermilk, salt, black pepper, onion granules and garlic granules. Add the chicken pieces, turning to coat. Cover and pop in the fridge for 2 hours or overnight.
In a shallow bowl, mix together the flour, salt, black pepper, paprika, oregano, cayenne pepper, garlic granules and onion granules. Remove the chicken from the marinade (let excess drip off chicken) and roll in the flour mixture. Ensure all sides are well coated.
Heat the rapeseed oil in a large pan over medium-high heat. Place the chicken fillets into the pan. Cook the fillets for 5-6 minutes per side or until the chicken is cooked all the way through.
Serve the crispy chicken fillet with tomato and lettuce or in the toasted bun for post workout meals. Serving suggestions below.