Portabella mushrooms can make good substitutes for hamburger buns. Here is a simple hamburger meal Rochelle and I whipped up in our cabin on our recent honeymoon. We just used one mushroom for the bottom "bun", but you could use two if you'd like it to look a bit more traditional.
Put the ground meat into a bowl, add the onion, garlic, pepper, and any other spices that you wish. Mix well, then form into three patties. Place the olive oil in a skillet, cook the patties, flipping often, until done to your liking. I like to flip the meat often to prevent any excessive browning, and serve it done rare. Set the burgers aside when done, covering with a plate so that they stay warm.
While the burgers are cooking, prepare the "buns" and any vegetables that you wish to top the burgers with. You'll want to cut the stems out of the mushrooms first, but you can save them to use in a different meal if you wish. We just had tomato and lettuce on hand to top these burgers with, but you can garnish your burgers any way you like.
Place the mushrooms into the skillet that you used to cook the burgers, cooking for around 2-3 minutes per side in the juices from the meat. Plate the mushrooms then add the meat and condiments of your choosing. A bit of steamed broccoli rounded out this particular meal.
Zone info: 3 servings at 6 protein blocks, 4 fat blocks (this will vary with your selection of meat), carbs are negligible.