First make the pickled cucumber. Place the cucumber slices in a bowl.
Heat the vinegar, sugar and salt in a microwaveable bowl/jug for 1 minute.
Pour the hot vinegar over the cucumber and allow to sit for 10 minutes.
Halve the chicken breasts horizontally so you have two thin fillets of chicken for each breast. Flatten the fillets slightly with a rolling pin or meat mallet.
Place the flour, eggs and crumb coating in three separate bowls.
Coat the chicken fillets first in the flour, then in the egg and finally in the crumb coating.
Heat the oil in a large frying pan.
Fry the chicken until golden brown and cooked through on both sides.
To serve, halve the rolls and add lettuce leaves and mayonnaise to each one.