Heat grill to high. Put the chicken breasts between 2 pieces of cling film and bash with a rolling pin until they’re about half their original thickness. Whizz the toast in a food processor to rough breadcrumbs, then tip out onto a plate.
Beat the egg and mustard together in a bowl and season. Dip the chicken into the egg, let the excess drip back into the bowl, then press into the toasty breadcrumbs. Put under the grill on a flat baking sheet and grill for about 10 mins, turning once, until golden and crisp on both sides and the chicken is cooked through.
Beat the lemon zest, juice and lots of black pepper into the mayonnaise. Spread some of the mayo onto the bottom halves of the buns, top with a chicken breast and salad of your choice and serve.