Place the fish fillets in a shallow dish. Add the onion, garlic, sugar, and salt to taste and rub to coat the fish (top and bottom).

Put the flour and Cajun Spice blend on a plate, mix them together and spread the mixture out on the plate. Place it near the stove. Heat the oil in a large frying pan over high heat until hot. Dip the fish in the flour and spice, coating it thickly, then gently add to hot oil. Reduce heat and cook, turning once until lightly browned, about 2 to 3 minutes per side. Remove and serve immediately.