1-First Step: Prepare the Onions Heat 2 tablespoons of oil in a large frying pan over medium heat. Add the sliced yellow onion, season lightly with salt, and sauté for 5 to 7 minutes until lightly browned. Lower the heat and cook, stirring occasionally, for 20 to 25 minutes until the onions are deeply caramelized and tender, then set them aside. For a low-calorie twist, use less oil here to keep things light.

2-Second Step: Form the Patties Take 2 pounds of ground beef and gently form it into four equal-sized patties, seasoning both sides with salt and pepper. Aim for about ¾-inch thickness and press a thumb into the center of each to prevent puffing during cooking. If you’re grilling, this step adapts well by preheating to medium-high heat for even results.

3-Third Step: Cook the Patties Heat the remaining 2 tablespoons of oil in a large carbon steel or cast iron skillet over medium heat. Add the patties and cook for 5 to 7 minutes per side for medium doneness, which hits around 140°F to 145°F internally. In the last 2 to 3 minutes, place 4 slices of Swiss cheese on top to melt perfectly; this works for dietary needs by swapping cheese if needed.

4-Fourth Step: Sauté the Mushrooms After removing the patties, add 2 cups of sliced mushrooms to the pan with the rendered beef fat, season with salt, and cook over high heat for 5 to 7 minutes until browned. Stir in 3 minced garlic cloves until fragrant, about 1 minute, to build that savory sauce base. For variations, you could add herbs if you’re customizing for flavor.

5-Fifth Step: Make the Sauce Pour in 2 cups of beef stock and let it simmer until reduced by half and slightly thickened, which takes about 10 minutes. Season with salt and pepper, and if desired, stir in 1 teaspoon of unsalted butter and minced fresh parsley for extra richness. This sauce can be adjusted for a vegan version by using vegetable stock instead.

6-Final Step: Assemble and Serve To finish, place the burger patties on the toasted bottom buns, layer with the caramelized onions and mushroom sauce, then top with the toasted buns. The total prep time is about 20 minutes and cook time around 35 minutes, making it quick for busy schedules. Serve immediately, and any extra sauce can be used for dipping rest the burgers for 5 minutes first to lock in juices.