Preheat your skillet or grill to medium-high heat, and add your vegetable oil to the pan if using a skillet.
Shape your ground beef into four equal patties, then sprinkle both sides with salt, pepper, and garlic powder.
Add the patties to the grill or skillet and cook for about 3-4 minutes per side, or until they reach your desired doneness.
While the burgers are cooking, melt the butter in a separate skillet and sauté the mushrooms for around 4-5 minutes until browned, then stir in Worcestershire sauce.
Place a slice of Swiss cheese on each cooked patty and let it melt for about 1 minute, then remove from the heat.
Toast your hamburger buns until golden, then mix mayonnaise and Dijon mustard together and spread on each bun.
Assemble each burger with a beef patty, sautéed mushrooms, and top bun.