Line a baking sheet with several layers of paper towels.
Prepare 1 bowl to fit inside another bowl that is filled with ice water (this will keep gluten from forming and will keep the batter crispy and light).
Whisk beer, flour, rice flour, baking powder, cayenne, salt, and pepper in smaller bowl to form the batter.
Heat oil in a heavy gauge pan to 360°F. Dredge onion strings in rice flour, a few at a time, then dredge in batter, and fry in hot oil until golden brown. Transfer to the prepared baking sheet and sprinkle with salt.