In a medium saucepan, bring the water to a boil. Stir in the quinoa and bring to a boil. Reduce the heat to low and simmer until the water is absorbed, about 30 minutes. (Just as with rice, do not stir—tilt the pan to see if there is any water left.) Set aside.
Meanwhile, trim and discard the parsley and mint stems. Chop, chop, chop the leaves until they are very fine, no piece larger than the head of a pin.
Transfer the quinoa to a bowl, fluffing with a fork. Stir in the chopped parsley and mint, arugula, carrots and tomatoes.
In a small bowl, whisk the lemon juice, oil, garlic, salt and cayenne. Pour over the quinoa and toss well. Season to taste with more salt and lemon juice, if desired. Cover and refrigerate for at least 30 minutes.