The genius in this sandwich is the addition of tangy unripe (green) mango, which is used here as a vegetable garnish. Pick a mango that is quite firm so you can easily cut it into small, thin matchsticks. Any leftover mango julienne can be refrigerated in an airtight container for several weeks. If the sandwich is made a few hours in advance, you might want to drizzle a little lime juice of olive oil on the avocado to keep it from discoloring. * Source: The Washington Post, adapted from Tom Colicchio's Wichcraft (Clarkson Potter, 2009)