*Buttermilk substitute: For 1 cup buttermilk, substitute 2 tablespoons lemon juice or vinegar plus enough milk to make 1 cup.
1. HEAT oil to 375°F in a deep fat fryer or heavy stockpot.
2. SLICE onion rings thinly, ideally using a mandoline or vegetable slicer.
3. PLACE onion rings in a large bowl, separated; cover with buttermilk.
4. COMBINE flour, salt, black pepper, cayenne, and 1/2 teaspoon of paprika in a medium bowl.
5. DRAIN onions from buttermilk and dredge in the seasoned flour. Shake off excess flour.
6. DEEP-FRY until crisp and golden, about 2 minutes. Drain on paper towels.
7. SPRINKLE with remaining paprika. You can serve the frizzled onions immediately, or keep at room temperature for a few hours.