Place the chicken breast between two large sheets of cling film or in a freezer bag or sandwich bag. Using a rolling pin, bash the chicken out until it is roughly 1cm thick. Remove the chicken and season it with the salt and pepper.
Bring a griddle or heavy-bottomed frying pan on a high heat. When hot, add the oil. Place in the chicken and fry it for 3-4 minutes on each side.
While the chicken is cooking, place the leaves, onion, ketchup and mayo on a slice of bread. When the chicken is cooked through, place it on top of the prepared leaves/sauces. Slap on another slice of bread and tuck in.
For lettuce don't make it complicated, just use whatever lettuce you have in the fridge, I used rocket just because it was there and it was fantastic.
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