2 tablespoons plus 1/2 cup all-purpose flour, divided
1 egg white
1/4 cup buttermilk
1/4 teaspoon salt
1/8 teaspoon cayenne pepper
Oil for deep-fat frying
Cut onion into 1/4-in. slices; separate into rings. In a shallow bowl, whisk 2 tablespoons flour, egg white, buttermilk, salt and cayenne until blended. Coat onion rings with remaining flour, then dip in buttermilk mixture.
In an electric skillet or deep-fat fryer, heat oil to 375°. Fry onion rings, a few at a time, for 1-2 minutes on each side or until golden brown. Drain on paper towels.