In a food processor, combine chopped mushrooms and walnuts. Pulse until finely chopped.

In a bowl, mix the mushroom-walnut mixture with breadcrumbs, egg, garlic powder, onion powder, salt, and pepper.

Form the mixture into patties.

Heat a skillet over medium heat and cook the patties for about 5-7 minutes on each side until golden brown.

Remove from skillet and set aside.

Place the cooked patty on a bun and top with cheese, lettuce, and tomato.

Serve immediately.