Traditionally this deep-fried battered fish would be served with 'chips' (fat french fries), but I actually prefer to do onion rings, because I use the same batter for both fish and rings.
Cut white fish into fillets and make sure there are no bones. (If you buy it it should already be this way, but if you caught your own then obviously you'll have to do it yourself!) I like to make them on the smaller side--I slice thick fillets in half depth-wise (so I have two thin fillets), and I cut them down to about 4x4inch pieces. I like a high batter-to-fish ratio. ☺
In a wide and/or shallow bowl, put 1 cup self-rising flour (or 1 cup white flour, 1 tsp baking powder, 1/2 tsp salt). Slowly add water until it is about the consistency of pancake batter...maybe 2/3 or 3/4 cup of water usually. If it is too thin it will all drip off the fish (or onions)!
Heat oil in a wok, saucepan, or deep frying pan (I like using a lid to avoid oil splatters, but an oil screen works too). You can deep fry the fish, but I find that using a high-sided frying pan I can 'deep fry' them in only about an inch of oil, I just have to turn the fish over halfway through the cooking. It uses less oil, and seems to get less oil into the fish as well.
Put battered fish pieces into oil and fry until golden and cooked through--about 3-5 minutes per side, depending on the size of the pieces and whether the fish was frozen or thawed. Use tongs or a slotted spoon to remove fish and set on a paper towel to drain. Serve hot!
*This does NOT reheat very well (it's ok in the oven but terrible in the microwave), so if you have leftovers I recommend eating them cold.