Place quinoa in a bowl and cover with cold water. Let sit for five minutes. Drain through a strainer and rinse until the water runs clear. Bring the 3 cups of water to a boil in a medium saucepan. Add salt (1/2-3/4 tsp) and quinoa. Bring back to a boil, and reduce the heat to low. Cover and simmer 15 minutes or until the quinoa is tender and translucent; each grain should have a little thread. Drain off the water in the pan through a strainer and return the quinoa to the pan. Cover the pan with a clean dish towel. Replace the lid and allow to sit for 10 minutes.
Meanwhile, place the finely diced cucumber in a colander and sprinkle with salt. Toss and allow to sit for 15 minutes. Rinse the cucumber with cold water and drain on paper towels. If using onion, place in a bowl and cover with cold water. Let sit for 5 minutes then drain, rinse with cold water and drain on paper towels.
Combine the tomatoes, chiles, cilantro, vinegar, lime juice, and olive oil in a bowl. Add the cucumber and onion, season to taste with salt and add the quinoa. Toss together and taste, adjusting seasonings if necessary.
Serve garnished with avacado slices and cilantro sprigs.