Step 1

Heat a skillet over medium heat. Add butter and oil; heat until butter is melted. Add onion and garlic. Cook until onion is soft and translucent, about 5 minutes.

Step 2

Meanwhile, divide ground meat into 4 portions and shape into patties. Season with salt and seasoning.

Step 3

Heat a large skillet or flat-top over medium-high heat for a few minutes. Place patties into the hot skillet and smash with a spatula or bacon press. Hold patties down for 30 seconds, then cover each patty with some of the cooked onion. Cook for 2 minutes. Carefully flip burgers, making sure to scrape the browned bits off the skillet.

Step 4

Press down on the flipped patties to sink the onions into the meat.

Step 5

Meanwhile, add sliced mushrooms to the skillet used for the onion and cook until softened, about 5 minutes.

Step 6

Season patties with salt and pepper and place a slice of cheese on each. Cook until burgers have reached the desired doneness, 2 to 4 minutes more. Top each patty with leftover onion and mushrooms and serve on buns.