Shake Shack's Shroom Burger is a crispy-fried portobello mushroom filled with melted Muenster and cheddar cheese, topped with lettuce, tomato, and ShackSauce on a potato bun. One of the best vegetarian burgers in fast food that doesn't try to imitate meat, it lets the cheese-stuffed mushroom be the star and has no real equivalent at any other chain.