Our onion rings, made from white onions, are fried in peanut oil.
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.
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.)
Use a slotted spoon to transfer rings to a paper-towel-lined baking sheet to drain. Season immediately with salt.
After cooking the onion rings, keep them warm in a 200 degree oven while you finish the remaining batches.