Heat the olive oil in a 12-inch nonstick skillet over moderately high heat for 1 minute. Add the chopped onion and garlic and sauté, stirring occasionally, until limp and golden- about 3 minutes.
Add the mushrooms and sauté, stirring now and then, until the mushrooms soften and brown- about 4 minutes. Transfer the mushroom mixture to a large bowl and cool for 10 minutes.
Using wooden spoon, mix salt, pepper and bread crumbs into the mushroom mixture. Add the egg whites and mix thoroughly.
Scoop up the mixture by ½-cup measure, then using hands, shape into four patties about ¾-inch-thick.
Wipe out the skillet, spray with nonstick spray, and set over moderate heat. Add the burger patties and brown about 3 minutes on each side.
Serve the mushroom burgers on buns (if desired) and add your favorite fixings and condiments.