Portabella Mushroom Sandwich

1 Package sliced Portabella Mushrooms1 small onion, sliced1 medium tomato, sliced lettuce1/8 cup mozzarella cheese (provolone or swiss can be used too)1/2 teaspoon mayo8 slices whole wheat breadcooking spraySpray skillet with cooking spray.Sautee mushrooms 5-6 minutes or until tender, turning occasionally.Onions can also be sauteed at this time if desired. Set aside. Set oven to broil. Toast 4 slices of the bread.Arrange bread on cookie sheet covered in foil.Spread bread with a thin layer of mayo.Next add the onions, sauteed or raw.Arrange the mushrooms on top of the onions.Add a tomato slice and a slice of cheese.Broil until cheese melts 1-2 minutes.Toast remaining 4 slices of bread.Top sandwich with lettuce.Add more mayo if desired to bread.Make 4 sandwiches.