Peel onions under running cold water. Cut into 1/4 inch slices and separate into rings. Heat oil (1 inch) in 3-quart saucepan to 375 F.
Beat remaining ingredients with hand beater until smooth. Dip each ring into batter, letting excess drip into bowl.
Fry a few onion rings at a time in hot oil, turning once, until golden brown, about 2 minutes; drain.