Whisk fish sauce to taste, lime zest, lime juice, oil, brown sugar, vinegar and crushed red pepper in a large bowl until combined. Add cucumber, avocado, mango and cilantro; gently toss to coat. Serve garnished with lime zest, if desired.
Fish sauce is a pungent Southeast Asian condiment; find it in large supermarkets and Asian markets. To peel and cut a mango: 1. Slice both ends off the mango, revealing the long, slender seed inside. Set the fruit upright on a work surface and remove the skin with a sharp knife. 2. With the seed perpendicular to you, slice the fruit from both sides of the seed, yielding two large pieces. 3. Turn the seed parallel to you and slice the two smaller pieces of fruit from each side. 4. Cut the fruit into the desired shape.
