First a bit of violence, also called tenderising. Place the chicken breasts between two sheets of parchment paper. With a tender hammer or rolling pin, strike the breasts until they are reduced in thickness by half.
Pull the bread into chunks and put into a food processor. Whizz the bread until they become breadcrumbs.
When whizzed, pour some into a wide bowl.
Crack the egg into a separate bowl and whisk until beaten. Add half of the mustard and whisk again until combined.
Messy part.
Dip the chicken breasts into the beaten egg shaking off any excess then dunk it into the breadcrumbs.
Add more breadcrumbs to the dish if needed. Any left over can be stored in an airtight container.
Slice the red onion and gherkins into thin slices and put to one side for toppings.
Add the breasts onto a slightly oiled baking tray to avoid sticking.
Place under the grill for 5 minutes then remove and turn over. Place back under the grill for another 5 minutes. The chicken should be fully cooked and golden brown.
Remove from the grill and add the slices of cheese and return under the grill for a minute or two for the cheese to melt.
Place the buns under the grill quickly to warm but not bake at the same time as the cheese.
Add the other half of the mustard into the mayonnaise and thoroughly mix. in a serving bowl.
I serve with the mayonnaise spread on the bottom part of the bun, added the cheesed breast topping with the gherkins and onions and another dollop of mustard mayonnaise.