Cut the onion in half and remove skin, thinly slice it. Heat a medium pan to medium heat, add 1 tablespoon butter and 2 tablespoons olive oil. Then add the sliced onions and 1 teaspoon salt. Cook for 15 minutes, stirring often.

While the onions cook, prep the mushrooms by removing the stems and scooping out the gills using a spoon. Using a pastry brush and brush on 1 tablespoon of olive oil on both sides. Season with ½ teaspoon of salt and garlic powder.

Bring another pan to medium-low heat, add 3 tablespoons of butter and melt it. Add the mushrooms top side up, press them down a bit. Cover with a lid and cook for 5 minutes.

Flip the mushrooms, divide the shredded gouda on the inside of each mushroom. Cover with the lid and lower the heat to low, cook for an additional 4-5 minutes. Top with the cooked onions and remove from heat.

Toast your buns, if desired. Spread the Truff spicy mayonnaise on both sides, add the arugula and top with mushroom burgers. Cover with the top of the bun and enjoy.