I remember being on the road with my father in rural BFE. We stopped at an indy diner on the outskirts of some one-stop-light town. He ordered a patty melt. I asked what it was, and after his description, ordered one for myself. It was perfect - the cook smashing out the burger patties to "fit" the rye bread which was buttered 'n' browned on the grill, the griddled onions, the white cheddar & Swiss cheeses, the house-made seasoning salt which was applied to both the burger patties and the french fries... I was in heaven. The root beer was very cold, served with crushed ice, and the waitress refilled my glass without adding to our bill (Dad noticed and bumped up her tip).
Memory's a funny thing. No patty melt since (even ones made at home) has come close.