1-Caramelize the onions: Melt butter with olive oil in a skillet over medium-low heat, add sliced onions and salt, and cook for 30-40 minutes, stirring occasionally, until deep golden brown.

2-Sauté the mushrooms: In a separate pan over medium-high heat, cook sliced mushrooms with oil until golden and all moisture has evaporated, then set aside.

3-Make the burger sauce: In a small bowl, mix mayonnaise, ketchup, Dijon mustard, garlic powder, and smoked paprika until smooth.

4-Form the patties: Divide ground beef into 4 equal portions and gently shape into patties slightly larger than the buns, handling the meat as little as possible.

5-Season the patties: Season both sides of each patty generously with salt and black pepper just before cooking.

6-Cook the patties: Heat a cast iron skillet or grill to medium-high, lightly oil, and cook patties for 4-5 minutes per side, flipping only once and never pressing down.

7-Melt the cheese: During the last minute of cooking, place a slice of Swiss cheese on each patty and cover to let it melt completely.

8-Toast the buns: Lightly toast the burger buns in a dry skillet or on the grill until golden and crisp.

9-Assemble the burgers: Spread burger sauce on both bun halves, then layer the patty, caramelized onions, sautéed mushrooms, lettuce, tomato, and pickles.

10-Rest and serve: Let the assembled burgers rest for 1-2 minutes so juices redistribute, then serve immediately.