1-First, prepare the ingredients by washing and slicing mushrooms, and seasoning the ground beef with salt and pepper.

2-Second, heat 2 tablespoons of oil in a large frying pan over medium heat; add onions, season with salt, and sauté for 5 to 7 minutes until lightly browned. Reduce heat to low and continue cooking while stirring occasionally for 20 to 25 minutes until deeply caramelized. Set onions aside for later use. For tips on this process, visit an external resource like How to Caramelize Onions.

3-Third, shape 2 pounds of ground beef into four equal patties and season both sides with salt and pepper.

4-Fourth, heat the remaining 2 tablespoons of oil in a large skillet or griddle over medium heat. Cook patties for 5 to 7 minutes per side for medium doneness, adjusting time as needed. Add 4 slices of Swiss cheese on top during the last 2 to 3 minutes and allow to melt. Remove patties and let rest.

5-Fifth, add 2 cups of sliced mushrooms to the same pan with beef fat, season gently with salt, increase heat to high, and sauté for 5 to 7 minutes until browned. Add 3 minced garlic cloves, stirring until fragrant. Pour 2 cups of beef stock into the pan, simmer until liquid is reduced by half and slightly thickened. Season with salt and pepper, then add optional minced fresh parsley and 1 teaspoon of unsalted butter.

6-Sixth, toast burger buns lightly on the grill or skillet for 1-2 minutes.

7-Final step: Assemble the burgers by placing a patty on the toasted bottom bun, topping with caramelized onions, then mushrooms with sauce, and finishing with the top bun. Serve immediately with leftover mushroom gravy if desired. Adaptations for dietary needs include using plant-based patties for vegans, gluten-free buns or lettuce wraps, and lean protein for low-calorie options.