First, make sure your fish is de-scaled and gutted and the gills removed. Also, trim the fins on the sides of the head. You can trim the tail a little bit if it is too big. Wash the fish thoroughly with water mixed with a bit of salt and vinegar or lemon. You can even immerse the fish in this salty vinegar/lemon water for about half an hour, it reduces any fishy smell. Drain and pat the fish dry. Make 2-3 slits on the fleshy part of the fish, on both sides at 2-3 inch intervals. This helps make sure your fish will get cooked properly all the way through.
Mix the salt, chilli powder, garlic, turmeric, cumin, tandoori and lemon juice to form a paste. Slather this marinade on both sides of the fish, making sure the marinade goes all the way into the flesh through the slits. Also rub the marinade inside the fish. Set the fish to marinate for about an hour which will allow the flavours to infuse into the fish.
Turn the grill on high. If you're using an oven, have it at 220 C. You can also use a charcoal grill for this. Whilst the bbq grill or oven is pre-heating, brush the 2 tbsp of oil on both sides of the fish, making sure a thin layer of oil covers every part of the fish. This will prevent the fish from sticking to the grill and also will help it to achieve a good colour during grilling.
Once the bbq grill is ready, wipe oil on it using a kitchen towel, then place the fish on it.
If you're baking in the oven, use the grill-like tray for this, brush it with oil and make sure to place something under it to catch the drippings that will fall as the fish bakes.
Give the fish 12-15 minutes, then gently slide a spatula under the fish and pry it loose if it is stuck. Be very gentle as fish has a tendency to stick to the grill, so do this patiently and gently. Then, using two spatulas, slowly turn the fish over and bake/grill again for another 12-15 minutes. Rule of thumb when grilling fish is it takes 10 minutes per inch of thickness overall. So if the fish is 2 inches thick, it will take a total of 20 minutes (10 per side). If it is thicker than 2 inches, adjust the time accordingly.
When done, remove the fish and again gently pry it off the grill and place on your serving platter. Garnish with slices of lemon or lime and potatoes, tomatoes, whatever you like. Tastes great with roast potatoes or gravy potatoes.