In a pot, add quinoa and water. Bring the mixture to a boil. After it reaches a boil, lower the temperature, place a lid on the pot, and let it cook for 12 minutes. Use a fork to fluff the quinoa, and let it cool for 10 minutes.
Place the tomatoes, cucumbers, bell peppers, and onions into a big bowl.
Put the cooled quinoa, feta, and mint in the bowl.
Mix together olive oil, lemon juice, salt and pepper in a small cup, and pour it over the salad to toss.
If you have some extra time, chill for 20 minutes.