Put mutton mince, Worcestershire sauce, garlic, salt, crushed peppercorns, thyme and egg in a bowl. Mix well and set aside for ten to fifteen minutes.
Divide mutton mince mixture into four equal portions and stuff each portion with a spoonful of grated cheese and shape into a patty.
Heat some oil in a non-stick pan. Place patties on it and shallow-fry, turning sides, till evenly golden on both sides.
Slit burger buns horizontally.
Heat a non-stick pan. Place burger buns on it, reduce heat and toast till crisp.
Combine mayonnaise, mustard paste, tomato ketchup and crushed peppercorns in another bowl and mix well.
Heat one tablespoon oil in another non-stick pan. Add onions and caramelize till golden.
8. Place each toasted burger bun base on a worktop. Spread a teaspoon of prepared mayo spread and on each and place one or two torn lettuce leaves on top. Put two to three tomato roundels on top, sprinkle salt, crushed peppercorns and place a patty on top. Finally top with some caramelized onions.
Combine five to six lettuce leaves and leftover mayo spread in a bowl and mix gently.
Place this lettuce salad on a serving plate. Cover the burgers with the remaining burger halves, press little and place along the salad along with gherkins.