Heat the oil in a deep frying pan or saucepan to 350 F. Alternately, use a deep frying machine set to 350 F.
To cut the onion rings, carefully trim the root ends of the onion, peel the outer layer, then slice rounds. Once you have the sections, carefully pop out the onion ring slices.
In a medium-sized mixing bowl, whisk together the flour, cornstarch, garlic powder, and salt.
Whisk in enough beer to make a smooth paste, then continue adding beer, gradually, until the batter is smooth (but still bubbly).
Using tongs, dip the onion rings, one at a time, in the batter.
Shake off the excess, and then fry in small batches for 4 to 5 minutes, turning occasionally until crisp and golden.