Add olive oil to heavy hot saucepan. Add shallots, garlic, mushrooms, saute until mushrooms are tender.
Add flour and cook about 1 minute then deglaze pan with Marsala wine.
Add vegetable stock and cook until thick and flavors are blended.
Turn heat to low while the burger is cooked.
Heat a cast iron skillet on medium high heat. Season the Omaha angus burgers with salt and pepper on both sides.
Add the burgers to the skillet. Cook 4 minutes on the first side. (You will know the burger is ready to flip when the skillet side down shrinks.)
After flipping the burger, cook for 2 minutes. Then, add 4 slices of the cheddar cheese and cover the pan with a top to get a really good melt.
Meanwhile, place the buns with the bottom bun covered with 4 slices of the Fontina cheese in a 325 degree oven.
Cook for 2 minutes to brown the bun and melt the cheese.
Assemble the sandwich by taking the Omaha angus steak burger on the Fontina cheese bun and smother the burger with as much of the marsala mushrooms as you dare.
Serve and enjoy!