1. Combine flour, cumin, cayenne, and 1 teaspoon salt in a medium bowl. Slowly whisk in buttermilk, beer, and egg until smooth. Let batter stand 15 minutes.

2. In a large saucepan, heat oil over medium-high heat until a deep-fry thermometer registers 375 degrees. Working in batches, dip onion slices in batter, turning to coat. Gently drop slices into hot oil. Cook, turning rings once, until golden brown, about 2 minutes. (Adjust heat between batches as necessary to keep oil at a steady temperature.)

3. Use a slotted spoon to transfer rings to a paper-towel-lined baking sheet to drain. Season immediately with salt.