Rinse the quinoa under cold water. Combine it with the water and a pinch of salt in a saucepan. Bring to a boil, reduce the heat, cover, and simmer for 15 minutes. Remove from the heat, rest covered for 5 minutes, then fluff with a fork.
While the quinoa cooks, dice the cucumber, halve the tomatoes and olives, drain and rinse the chickpeas, and chop the parsley and mint.
Whisk the olive oil, lemon juice, Dijon mustard, grated garlic, oregano, salt, and black pepper in a small bowl.
Let the quinoa cool until warm or room temperature. Add it to a large bowl with the chickpeas, cucumber, tomatoes, olives, and herbs. Pour over the dressing and toss gently.
Fold in the feta. Taste and adjust the seasoning. Chill for 15 minutes if time allows, or serve at room temperature.