Toast the cumin, fennel and ajowan in a small pan until fragrant. Crush the spices in a small mortar and pestle or spice grinder. Mix with the chiles, Tandoori seasoning, lemon juice and yogurt.
Spread over the fish and leave for 20 30 minutes to absorb flavors.
Cook the fish on barbecue grill or under a moderate broiler until done, about 6 minutes on each side.
Baste with a little oil during cooking to prevent the fish from drying out.
Sprinkle with pepper and a little lemon juice before serving.
Serves 4 - 6