Place buttermilk and onions in a shallow dish and let soak for 1 hour.
Combine flour, salt, garlic powder, black pepper, and cayenne pepper in a bowl and set aside.
Heat about 2 inches of oil in a high-sided pot to 375 degrees. Working in batches, dredge a handful of the buttermilk-soaked onions in the flour mixture, shake off any excess flour, and carefully drop into the hot oil. Cook onions until golden brown, about 4-5 minutes, stirring occasionally. Remove onions onto paper towels to drain and sprinkle with a pinch of salt while onions are still hot. Repeat with remaining onions.
NOTES
- Serve crispy onion strings on casseroles, burgers, salads, or by themselves with a dipping sauce.