My friend Susan Benett taught me how to cook a better portobello mushroom: remove the black insides. Without them, the mushroom stays drier (instead of waterlogged) and better able to hold ingredients. This burger is perfect for the mushroom lover who doesn't want all the carbohydrates found in bread. This calls for making an open-faced burger, but you can cook another four mushroom caps and place them on top, if desired.