First, combine your beef and roasted mushroom base in a large bowl. Once these two elements are evenly combined, slowly add in your chopped onions, salt and pepper. Finally crack your egg in the bowl over the rest of your burger base and stir it in along with your Westercher sauce.
Your next step is to take your burger base and form it into 6 or 8 individual patties and put them on a non-stick baking sheet. Refrigerate your patties for 30 minutes.
Cook your patties for 4 minutes on each side in a large skillet filled with enough oil to coat the bottom.
Finally, bake your patties for five minute on a parchment-lined tray. Once finished, remove from the oven, place the cheese slices on each burger and let sit for a couple of minutes. Finally, serve with your favorite toppings!