My friend Gigi posted an amazing recipe for Portabella Mushroom Burgers on her blog at http://www.gigikkitchen.com/search?q=mushroom.
I've been making this recipe very often since then, and my only changes are that I sometimes use pita or no bread at all, and I swap the cheddar for mozzarella because I love the stretchy gooeyness of mozzarella.
The sauce she recommends really adds a lot to the recipe, but if you don't have it, don't worry. It's quite delicious without it, too.
Happy printing and happy cooking!
Download and print the recipe here