Hamburger Patty Melts

I've made patty melts on white bread and rye bread,
with Jack cheese or Swiss cheese.
I think the classic is rye bread and Swiss cheese,
which is my favorite, but I'd venture to guess
that varies a bit in different parts of the country.

Ingredients:
2 small onions or 1 medium to large onion, peeled and sliced
a dab of olive oil
a dab of butter
2 1/4 pound hamburger patties
4 slices rye bread
2-3 slices Swiss cheese
another dab of butter for the outside of the sandwiches

Directions:
Heat the olive oil and a dab of butter
in a frying pan while slicing the onions.
Add the onions to the pan and cook slowly,
stirring now and then, until golden and soft.
Cook the hamburger patties either in the same skillet
(after removing the onions), in a second skillet.
If your patties are frozen and thin,
this takes about 4 minutes per side.
When they're done, wipe out the pan or grill.
Assemble the sandwiches with the rye bread, a burger,
Swiss cheese and grilled onions.
Top with another slice of rye bread
and butter the outside of the sandwiches.
Grill like you would a grilled cheese sandwich,
just until the bread is lightly browned and the cheese is melted.