We get those big mushrooms that are essentially overgrown button mushrooms, dab a bit of olive or grapeseed oil on, and broil them until juicy, about five or ten minutes.
Meanwhile, I sauteed half a large sweet onion. We ate our burgers in portuguese buns fresh from the local bakery. Sides included baby gherkins, calamata olives, raita, and a salad of celery apple carrot and roasted almonds.
What a great meal; easily digestible by our evening taekwondo workout, and pretty healthy.