You can do this in a charcoal grill or on the hot coals of a campfire!
1. Clean and rinse whole fish in cold water. Stuff with lemon wedges and onion slices.
2. Wrap fish in aluminum foil. Lay the foil-wrapped fish directly on hot charcoal or hot coals. Cook 7 minutes; turn the fish over and cook 6 or 7 minutes on the other side.
3. Carefully remove fish (still in foil) from the coals (use tongs or cooking mitt -don't want to get burnt). Let cool for a few minutes.
4. Open foil, season with salt and pepper to taste, and enjoy.
Cooking tip: To remove the bones entirely (so you don't need to eat around them) using forks, just pull the the fish's back meat in one direction and the belly meat in the other, and you can remove the bones in one piece.