In a large saucepan, mix the quinoa and water and bring to a boil over medium heat. Reduce heat and allow the quinoa to simmer until the water is absorbed, about 15-20 minutes.
Remove from heat, stir gently, and allow to cool while you prepare the remaining ingredients.
In a large bowl, stir together the parsley, mint, chopped tomatoes, diced onion, diced cucumber, and lemon or lime juice.
Season with salt and pepper to taste and add more lemon juice if needed.
When the quinoa is cooled, gently stir it into the mixture in the bowl.
Refrigerate for an hour to allow the flavors to blend if possible.
Serve garnished with sliced avocado, if desired.