Place the avocado, water, lime juice, olive oil, agave nectar, garlic, and salt in a blender. Purée until completely smooth. Add the cilantro and pulse until the leaves are broken up into smaller pieces. Transfer the dressing to a squeeze bottle or jar, and refrigerate.

In a large pan, warm the oil over medium-high heat. Once hot, add the red onions, poblano pepper, bell peppers, and jalapeño to the pan. Sauté for 5 to 7 minutes, browning the pepper mix on a couple sides and stirring occasionally. Once the onions are nearly translucent, add the mushrooms to the pan. Continue to sauté for 5 minutes, or until the mushrooms have decreased in size and are browned on the edges. Season with salt and pepper.

Divide the lettuce between 4 plates, then top with black beans and fajita veggies. Drizzle dressing over the salad, or serve it on the side. Garnish with the optional dried sweet corn, and serve.