Place mushrooms in a food processor fitted with a metal blade. Pulse until finely chopped.
In a large bowl, combine mushrooms, beef, onion, garlic, breadcrumbs and egg; mix all ingredients until incorporated. Form into six patties and cook either on the barbecue or the stovetop until internal temperature reaches 160°F.
During last minute of cooking, top patties with cheese. Keep warm until cheese begins to melt.
Place patty on warmed bun and top with desired toppings.
