2cupsgrated cheese I used Gruyère and sharp cheddar
butter
Instructions
Split the beef into 4 equal balls then flatten with the palm of your hand. I try to get a patty roughly the same shape as the bread I'm using.
Season generously on both sides with salt, pepper and garlic powder (optional).
Cook the patties in a large skillet for 5-6 minutes per side until deeply golden and cooked through. Remove from the pan and set aside.
In the same pan you cooked the patties in, melt a tablespoon of butter.
Add thinly sliced onions and mushrooms and cook for 20-30 minutes until the onions are caramelized.
I added a splash of Worcestershire sauce to the onions while they were cooking but this is optional.
Reduce the heat if the vegetables are darkening too quickly. You want them to be soft and silky with a deep caramelized flavor. Set aside.
Assemble the sandwiches by layering the beef patties, a generous spoonful of the onions and a few slices of cheese in between two slices of bread.
Butter the outsides of the sandwiches then cook in a pan set over medium heat until golden brown on both sides and hot throughout. Covering the pan with a lid while cooking will speed up the melting of the cheese.
Once ready, remove the patty melt from the pan, slice and serve.