To make the caramelised cabbage, place the butter in a saucepan over high heat. Add the cabbage and cook, stirring, for 5 minutes or until softened. Add the sugar and the vinegar and cook, stirring, for 10 minutes or until most of the liquid is evaporated and the cabbage is tender. Set aside.
Spread the bread with butter on one side and mayonnaise and mustard on the other side. Slice the corned beef and place on half the bread slices, butter-side down. Top with cheese and cabbage and sandwich with remaining slices. Heat a large non-stick frying pan over high heat. Cook the sandwiches, in batches, for 2–3 minutes each side, turning carefully, or until golden and the cheese is melted. Serve with the gherkins. Makes 6.