We had pattymelts for dinner and this is how I "built" them.
I started by caramelizing a sliced sweet onion. My secret is a pinch of brown sugar in with the butter in the pan. Set the onions aside.
Four slices of caraway bread, buttered on both sides. Grill then set aside.
I made the patties by putting them into a zip-bag and flattening. Cook until done, adding Swiss cheese near the end.
Layer the sandwich, add the onions, mustard, horseradish. These were almost too much.