1In a mixing bowl, combine ground beef, egg, italian seasoning, gravy mix, garlic powder, all-purpose seasoning, worcestershire sauce, and crushed crackers, then mix well.
2Shape the meat mixture into large patties.
3Heat bacon grease in a skillet and sear the burger patties on both sides until browned; then remove and place in a baking dish.
4In the same skillet, melt butter and add chopped mushrooms and thinly sliced yellow onion, sautéing until the onions are translucent.
5Stir in chicken broth, worcestershire sauce, and salt, then bring the mixture to a simmer.
6In a jar, combine corn starch and heavy cream, secure the lid, and shake vigorously to create a slurry.
7Pour the slurry into the simmering mushroom mixture and continue to simmer until the sauce thickens.
8Pour the prepared sauce over the burger patties in the baking dish, top each patty with a slice of white american cheese, cover with foil, and bake at 350°F for 30 minutes, then uncover and bake for an additional 30 minutes.