Portabella Mushrooms make great vegetarian burgers due to their big size. Clean and remove stem and brush them with a big of olive oil and bake them for about 15 -20 minutes in at 350 degrees. Let them sit a bit before making your burger as they tend to be a bit wet when they come out of the oven. Use them as you would normally use a regular burger.
Paired with oven yam fries and steamed vegetables....it makes a great low fat tasty meal.