Cut the fish into 1 cm (½ inch) strips and season to taste. Place the cornflour, egg and breadcrumbs in 3 separate shallow bowls, then dip the fish pieces first in the cornflour, then in the egg, and finally in the breadcrumbs to coat them and spread out on a large plate or baking sheet.
Meanwhile, heat the oil in a deep fat fryer or deep, heavy-based saucepan to 180˚C (350˚F), or until a cube of bread browns in 15 seconds.
Cook the fish in the hot oil for 3–4 minutes until golden brown, then remove with a slotted spoon and drain on kitchen paper. Mix the mayonnaise with the wasabi. Serve the crispy fish immediately with the spicy mayonnaise and some lemon wedges.