Rinse quinoa thoroughly under cold water for 30 seconds. Cook with vegetable stock and salt over low heat, covered, for 13 to 15 minutes until liquid is absorbed.
Remove from heat and steam covered for 5 minutes. Fluff with a fork and spread on a tray to cool completely.
Whisk all dressing ingredients together until emulsified. Taste and adjust until bright, tangy, and herby.
Soak diced red onion in cold water for 5 minutes. Drain and pat dry.
Dress the cooled quinoa with half the dressing and toss well before adding anything else.
Add cucumber, tomatoes, red onion, olives, bell pepper, and chickpeas. Toss to combine.
Pour remaining dressing over the salad. Toss and taste. Adjust seasoning.
Fold in feta, spinach or parsley, and mint gently at the very end.
Finish with extra feta, fresh mint, and a drizzle of olive oil. Serve at room temperature or slightly chilled.