Prepare the Sautéed Mushrooms:

In a skillet over medium heat, melt the butter and olive oil together.

Add the sliced mushrooms and cook, stirring occasionally, until they are browned and tender, about 5-7 minutes.

Add the minced garlic, salt, pepper, and thyme (if using), and cook for an additional 1-2 minutes, until fragrant. Remove from heat and set aside.

Prepare the Burger Patties:

In a mixing bowl, combine the ground beef, Worcestershire sauce, garlic powder (if using), salt, and pepper. Mix gently until just combined; avoid over-mixing to keep the patties tender.

Divide the mixture into 4 equal portions and shape each portion into a patty, slightly larger than the buns as they will shrink when cooked.

Cook the Burger Patties:

Preheat your grill or skillet over medium-high heat. Lightly oil the grill grates or skillet.

Cook the burger patties for about 4-5 minutes on each side, or until they reach your desired level of doneness (160°F for medium).

During the last minute of cooking, place a slice of Swiss cheese on each patty and cover the skillet or close the grill to melt the cheese.

Toast the Buns:

While the burgers are cooking, lightly toast the hamburger buns on the grill or in the skillet until golden brown.

Assemble the Burgers:

Place the cooked burger patties with melted Swiss cheese on the bottom halves of the toasted buns.

Top each patty with a generous portion of sautéed mushrooms.

Add lettuce, tomato, and onion slices if desired.

Spread condiments like mayonnaise, mustard, or ketchup on the top halves of the buns, if using.

Close the burgers with the top buns.

Serve:

Serve the Mushroom Swiss Burgers immediately, paired with your favorite side dishes like fries, salad, or chips.