Season the ground beef with salt, pepper, and any additional seasonings you prefer. Form the beef into 4 patties, making them slightly larger than the buns to account for shrinkage while cooking.
In a skillet, melt a tablespoon of butter and sauté the mushrooms over medium heat until they’re golden and tender, about 5-7 minutes.
Cook the patties on a grill or stovetop over medium-high heat for 4-5 minutes per side, or until they reach your desired doneness. Place a slice of Swiss cheese on each patty during the last minute of cooking to allow it to melt.
Toast the burger buns lightly. Assemble the burger by placing the cooked patty with melted cheese on the bottom bun, topping with sautéed mushrooms, and adding your favorite condiments. Place the top bun on and serve immediately.