The key to a successful patty melt is in the assembly. You want a juicy, messy, melty experience, but you want it to hold together. It’s a bit of a process, but well worth the effort for as often as I make this decadent sandwich–maybe once or twice a year!
Heat oven to 375. Start with your favorite patty. I like Gardein because it has a firm texture that holds up well. Brown it up in a skillet with a bit of oil and salt and pepper, then set aside and keep warm.
In a separate skillet, and simultaneously with the patties if possible, brown sliced onions until well caramelized. Season with salt and pepper and set aside and keep warm. In the same skillet, sauté and season mushrooms with salt and pepper and a couple tablespoons of worcestershire sauce. Keep warm.
When the oven is hot, toast buttered slices of bread on a sheet pan (butter side down) for about 5 minutes, until a bit crisp on the bottom. Add slices of your favorite melty cheeze ( I use Daiya slices) and return to the oven about 4-5 more minutes, until cheeze is melted and bread is crisp and brown on the bottom.
Assemble the sandwich carefully while everything is still hot. This takes a bit of organization.
Cut and serve immediately.