For pickle, place cucumber in a small bowl, sprinkle with salt, set aside for 10 minutes and drain excess liquid.
Stir through sugar and vinegar and chill in the refrigerator for 30 minutes.
Mix together wasabi mayonnaise ingredients in a small bowl.
Heat oil in a deep fryer or large pot (filled halfway) to 180°C.
Meanwhile, sift together plain flour and cornflour in a medium bowl.
Add soda water and water to another medium bowl, whisk in the egg, removing any froth that develops with a small sieve.
Add flours in three stages, incorporating with chopsticks in a crisscross motion, being mindful not to over mix (small lumps in the batter are ok).
Working in batches, dip the vegetables into the batter, scraping off excess (you only want a thin coating) and fry for 3-4 minutes. Allow to drain on a wire rack set over a tray.
Serve tempura with wasabi mayonnaise and pickled cucumber on the side.
