Portabello mushrooms make for a healthy veggie burger option for a vegetarian barbecue or vegan cookout. This recipe tops pan-fried or grilled portobello mushrooms with a perfectly cool (but quick to whip up) avocado and yogurt sauce. For a vegan and non-dairy portobello mushroom burger, omit the yogurt sauce or make it with non-dairy soy or coconut milk yogurt. Avocados from mexico provided this recipe, which is perfect for a hot summer evening. You can include your favorite condiments or additions such as lettuce, fresh spinach leaves, or sliced tomatoes. Stack it up or keep it simple.