200ml beer or just enough to make a thick yet runny consistency
Wash the prawns well. Drain, then pat-dry completely. Cut off caps and ends of king oyster mushrooms and tear into thick strips.
Combine batter ingredients in a mixing bowl. Mix gently and quickly into a smooth batter. Let it stand in the refrigerator for at least half an hour.
Heat enough oil in a wok until just hot over medium heat. Dust prawns with plain flour and coat with the batter, then deep-fry until lightly golden. Dust sweet potato slices and mushroom strips with plain flour and coat with batter. Deep-fry over medium heat until crispy and golden in colour.