1.Shell and devein prawns, leaving tails intact. Make three small cuts on the underside of each prawn, halfway through flesh, to prevent curling when cooked.
2.Halve onion from root end. Push four toothpicks, at regular intervals, through each onion half to hold rings together; cut in between toothpicks.
3.Make tempura batter by whisk egg and soda water in large bowl until combined. Add sifted flours whisking lightly until combined (batter will be lumpy). Do not over whisk.
4.For lemon dipping sauce, stir vinegar, sugar and sauce in small saucepan until sugar dissolves. Remove from heat, add rind; stand 10 minutes. Strain sauce into serving dish; discard rind. Sprinkle with onion.
5.Heat oil in deep-fryer. Dust prawns, onion, fish, capsicum, kumara and zucchini lightly in flour; shake off excess flour. Dip, piece by piece, in batter; deep-fry until crisp. Drain on absorbent paper.
6.Serve tempura with dipping sauce and lemon wedges.