Place the grill in the centre of the oven. Pre-heat the oven on broil.
In a large non-stick skillet over high heat, quickly sear the meat in the duck fat, pulling it apart with a wooden spoon. Add salt and pepper. Set aside on a plate.
In the same skillet, brown the mushrooms and onion. Add duck fat as needed. Add salt and pepper. Return the meat to the skillet. Add the stock and sauce, and mix well. Adjust seasoning to taste. Sprinkle the cheese on top and let it melt. Remove the skillet from the heat.
Place open buns on a baking tray. Toast the buns in the oven, 1-2 minutes per side.
Place a bun on each plate, garnish with lettuce and divide the meat mixture equally on the buns. Pour cooking juices on top.
Recipe from David Vinas, professional chef