Basically a patty melt is just a hamburger with Swiss cheese and caramelized onions on rye bread with mayo.
I used Beyond Beef, Violife slices, vegenaise, and melt butter, and I made a loaf of white bread instead of rye bread (just didn't have time to source rye flour, etc.).
For the hamburger adding onion powder, garlic powder, black pepper, and a little vegan Worcestershire sauce works well.
If you want to spruce up the sauce a little instead of just mayo, you can add a little ketchup, dijon mustard, a little garlic powder and cayenne pepper.
Personally I would have enjoyed this more with some grilled broccoli rabe or pickled peppers, but that would have gone even further from being a patty melt.