6.
Place
a
slice
of
cheese
on
each
patty
and
let
it
melt
slightly.
7. Heat the melted butter in a large skillet and toast the hamburger buns. Place a patty on the bottom half of each bun, then top with tomato, onion, and lettuce. Spread the top half of each bun with the mustard and finish the burgers. Serve hot.
Serves 4