Whisk beer (or sparkling water) in a bowl with the rice flour until smooth. Add salt and let the mixture sit at room temperature for 10 minutes.

Heat oil to 375F/190C. Dredge mushrooms in batter, coating completely, and letting any excess drip off. Deep-fry, turning once, until golden (about 3 minutes).

Transfer to paper towel. Season with seasoning salt.

Serve with soy sauce or your favourite aioli.