In a large frying pan over medium heat, add 2 tablespoons of oil and heat for 1 minute.
Place in the onions, gently season with salt, and sauté for 5 to 7 minutes or until lightly browned. Turn the heat down to low and cook while occasionally stirring for 20 to 25 minutes until they are well browned and tender. Set aside.
Meanwhile, form the ground beef into 4 equal-sized patties. Season each patty generously with salt and pepper on both sides.
In a separate large carbon steel, cast iron skillet, or griddle, add the remaining 2 tablespoons of oil and heat for 1 minute.
Carefully place in the burgers and cook for 5 to 7 minutes per side for a medium burger, or until the desired internal temperature is achieved. Place Swiss cheese onto each burger 2 to 3 minutes before they are done cooking. Set aside.
In the same pan, add the sliced mushrooms to the rendered beef fat, gently season with salt, and turn the heat to high. Sauté for 5 to 7 minutes or until well browned.
Add the minced garlic and stir until fragrant. Pour in beef stock and cook until the liquid reduces by half and thickens.
To assemble, place a patty on a toasted bottom bun, add caramelized onions, followed by sautéed mushrooms and sauce. Top with the toasted bun.
Enjoy your Mushroom Swiss Burger with any leftover mushroom gravy on the side for dipping.
