Cook the quinoa: Bring 2 cups of water to a boil over high heat in a medium saucepan. Add the quinoa and 1/2 tsp salt, reduce heat to low, cover, and simmer until quinoa is tender, for about 15 minutes. Remove from heat and let stand, covered, for 5 minutes—fluff with a fork. Leave it to cool down completely before adding it to the salad.
Finely chop the parsley by making a bouquet of them, cut off the stems and then chop them all together using a sharp knife, then chop again until they are minced very well.
Finely chop the tomatoes and onions, then transfer them to a large salad bowl.
Add the cooked quinoa, dry or fresh mint, salt, black pepper, ground cumin (optional), freshly squeezed lemon juice, and olive oil. Give it a nice mix, and serve immediately or refrigerate for 3-4 days.