In a large bowl, combine ground beef, kosher salt, black pepper, garlic powder, and Worcestershire sauce. Mix until just combined, being careful not to overwork the meat.
Divide the mixture into 4 equal portions and shape each into a patty, about 3/4 inch thick.
Preheat your skillet or grill over medium-high heat.
Place the burger patties on the skillet or grill and cook for about 4-5 minutes per side for medium-rare, or until they reach your desired level of doneness.
In the last minute of cooking, place a slice of Swiss cheese on each patty and cover the skillet with a lid or close the grill to melt the cheese.
While the burgers are cooking, melt butter in a separate skillet over medium heat.
Add sliced mushrooms and thyme, seasoning with salt and pepper.
Sauté the mushrooms until they are golden brown and tender, about 5-7 minutes. Stir occasionally to ensure even cooking.
Place each cooked, cheese-topped patty on a lettuce leaf.
Top with a generous spoonful of the sautéed mushrooms.
Add a tomato slice and your choice of keto-friendly sauce if desired.
Wrap the lettuce around the contents to form a bun-less burger.