Put the steak between two sheets of cling film and beat with a rolling pin until half its original thickness. Thinly slice the steak, then mix with the teriyaki marinade in a bowl. Leave to marinate for 5-10 mins.
Roughly dice the cucumber and mix with the chopped coriander and lime juice. Season with a little salt.
Heat a frying pan until very hot, then fry the steak slices for 1½-2½ mins for rare to medium, turning the slices halfway through.
Pile the cucumber mixture into the lettuce leaves, then top with the seared teriyaki beef, chilli and red onion.