Heat the oil and butter in a pan until smoking. This is best done in a well-ventilated room.
Pat the fillets dry with paper towel and sprinkle spices over the fish.
Place two fillets in the pan at a time, turning to blacken both sides.
Repeat with the remaining fillets.
Prepare the couscous according to the packet instructions.
Add the tomatoes and rocket and mix well to combine.
Divide the couscous among the serving plates, and place a piece of fish on top.
Serve with lemon or lime wedges.
Serve the fish with oven-roasted baby potatoes and a salad or try it with roasted seasonal vegetables.