In a large bowl, cover the onion slices in buttermilk. Dredge through a mixture of flour seasoned with salt pepper. Fry in a deep pan filled with oil.
To prepare the Steakhouse Sauce, combine all ingredients thoroughly.
Preheat oil in a saut pan. Add mushrooms to pan and season with salt and pepper. Cook over medium heat. Allow to mushrooms to brown, then reduce and caramelize until there is a rich brown coating. Remove mushrooms from heat and set aside.
Heat a large skillet with oil to medium. Place beef patties onto skillet. Season with salt and pepper, and cook for 3 minutes; flip and then cook for one minute. Top burgers with mushrooms and 2 slices of Swiss cheese. Continue to cook for 2 more minutes for medium-rare doneness. Cover with a lid for the last 30 seconds to melt the cheese.
To assemble the burgers, place one patty on a toasted bun bottom. Top the patty with the onion straws and a dollop of the Steakhouse Sauce. Cover with the bun top. Repeat with remaining ingredients. Wrap the burgers in wax paper and serve.