1. Form the meat into 4 equal square-shaped patties and season on both sides with salt and pepper. (Try to shape them in squares similar to the shape of the bread, not to thick and large enough as they will shrink)
2. Heat your flat top or a large cast iron pan over medium-high heat. Working 2 patties at a
time, add to the pan and cook until a browned crust forms, about 3 minutes. Flip and continue to sear until cooked to medium well, about an additional 2 minutes. Remove from the pan and set aside, covered on a plate. Repeat with the remaining patties.
3. While the patties cook, cook the mushrooms and onions
4. Add a Tbsp of butter to a pan and grill the onions. Stirring occasionally for 10 minutes until onions are grilled. Set onions aside
5. Add a Tbsp of butter to another pan and grill the mushrooms until browned. Stir occasionally for 10 minutes. When the mushrooms are almost ready season with salt & pepper. Set aside
6. Wipe the flat top or cast iron pan to remove as mush grease as possible from the patties.
7. Heat flat top or cast iron pan to medium high. Butter the outside of your bread slice and
place buttered side down on the grill. Next add a small layer of cheese, place your burger patty, top with grilled mushrooms and grilled onions. Add a small amount of arugula and top with shredded cheese. Spread chipotle mayo or burger sauce on the inside of your top bun and close the sandwich. Using a weight or heavy cast iron pan, press down on the patty melt. Cook until the bottom is golden brown. Butter the outside of your top bun and flip the sandwich -just like you would for a grilled cheese. Weight down again until the cheese is melted and sandwich is golden brown. -You can work in batches or make more than one sandwich at a time depending on the space available.
8. Cut the patty melts in half and serve with a side of pickles.