1Sift the flours and bicarbonate of soda into a medium bowl. Combine the water and egg together in a jug then pour into flour mixture. Briefly whisk until almost combined but still lumpy (do not overmix). Stir in the crushed ice.
2One-third fill a saucepan or wok with oil and heat over medium heat until hot (a cube of bread should turn golden in 10 seconds).
3Dip mushrooms slices 1 at a time, into the batter to evenly coat, allowing excess to drain back into bowl. Drop into the oil and cook in small batches for 3-4 minutes or until batter is light golden. Use a slotted spoon to transfer mushrooms to a wire rack over a lined baking tray. Keep warm in the oven while cooking remaining mushrooms.
4Serve with tempura dipping sauce.
Tip: Tempura dipping sauce is available in bottles in the Asian section of large supermarkets. Alternately to make your own: combine 1/4 cup water, 2 tbs mirin, 2 tbs light soy sauce, 1/4 tsp finely grated lemon rind and 1/2 tsp finely grated ginger together in a jar, shake well and set aside for 30 minutes.