Heat a frying pan on medium heat. Spread butter evenly on one side of each bread slice; place two slices, butter side down, in the pan. Top with leeks, half of the cheese, the tomato slices, then the remaining cheese. Spread the mayonnaise on the unbuttered side of the remaining two bread slices; place mayonnaise side down on the sandwich. Toast until golden brown, then flip, and brown the other side.