In a large bowl, combine the ground beef, salt, pepper, garlic powder, and Worcestershire sauce. Mix gently to avoid overworking the meat, then divide into four equal portions and shape into patties.

Heat a skillet over medium heat, add a tablespoon of butter and olive oil. Sauté sliced mushrooms and onion for about 8-10 minutes until golden brown and tender, adding garlic and thyme for the last couple of minutes.

Increase the heat to medium-high and cook the patties for 4-5 minutes on each side, checking for desired doneness.

In the last minute of cooking, place a slice of Swiss cheese atop each patty and cover the skillet to melt the cheese.

Toast the burger buns cut side down in a separate skillet or on the grill until golden brown and crispy, about 2-3 minutes.

Assemble the burger with the cheesy patty, sautéed mushroom mixture, and any additional toppings you desire, then top with the other bun half.