Combine onion, spice mixture, garlic, salt, and 2 tablespoons oil to form a paste. Smear paste over the fish, to cover completely. Refrigerate, covered for 1 hour.
When ready to fry, dredge fish pieces in buttermilk and then the flour.
In a large nonstick frying pan over a medium heat, heat 2 tablespoons of oil until very hot. Place fish pieces in oil without crowding (fry in two batches if necessary, adding more oil if needed). Fry, turning once, until golden brown and cooked through.