For the sautéed black truffle onions:
In a large skillet melt the truffle butter over medium-low heat. Add the onions, black truffle and mushrooms. Cook, covered for 12 to 14 minutes or until onions are tender and translucent.
Uncover; increase heat to medium-high cook and stir for 3 to 5 minutes more or until onions are golden brown.
For the sautéed mushrooms:
Thinly slice mushrooms vertically.
Cook the mushrooms and truffle butter in a single layer over medium-high heat.Turn the mushrooms over a couple of times so that each side is browned and a bit crispy and chewy. Sprinkle mushrooms with salt and pepper taste.
For the burgers:
Season both sides of steakburgers with gunpowder seasoning and cracked black pepper. Grill burgers according to package. Melt cheese the last minute of grilling. Grill hamburger buns and assemble burgers with bottom of buns, cheeseburger, mushrooms, sautéed onions, cheeseburger, mushrooms, sautéed onions and top of buns.