Put the mince in a bowl and add a splash of Worcestershire sauce, the mustard, garlic salt and pepper. Mix well and then divide into balls. Take each ball and shape into thin patties. Make them bigger than your bun as they will shrink when you cook them. Stack the cheese slices in the middle of a patty. Put the other patty on top then pat down and pinch the edges so the cheese is completely enclosed. Heat a frying pan. Oil and season the burgers, then add to the pan and fry for five minutes each side. Put some lettuce and tomato on the base of each bun. Sit the burgers on top, and then add onion and gherkins to finish.