Rinse quinoa. Add to a medium pot with the water and 1 teaspoon of salt. Bring to a boil; reduce heat, cover, and simmer for 15-20 minutes or until cooked through. Remove from heat and set aside.
Using a large knife, smash the garlic clove. Peel, roughly mince, then sprinkle with 1/4 teaspoon of salt. Use a fork to smash the salt and garlic together, creating a paste. If this seems difficult at first, allow the salt and garlic to hang out for a few minutes and try again. (It does not need to be a super smooth paste, you just want most of the large bits broken up.)
In a small bowl, whisk together the tahini, lemon juice, zest, garlic and olive oil. The mixture will be very thick - add hot water 1 tablespoon at a time until it comes to a consistency somewhat like that of yogurt. Give it a taste to check for seasoning, adding more salt if needed.
In a large bowl, toss the cooked quinoa with the cucumber and tomatoes. Add in the dressing, stirring to coat. Cover and chill before serving to allow the flavors time to meld. Will keep, covered in the refrigerator, for 3 days.