Trim stem ends of mushrooms 1/4 inch below caps.
If mushrooms are large, cut into halves.
Heat oil (1 inch) to 360°.
Beat remaining ingredients with hand beater until smooth.
Dip mushrooms into batter, letting excess batter drip into bowl.
Fry mushrooms, a few at a time, until golden brown, turning once, about 2 minutes.
Drain on paper towels.