Rinse quinoa and combine with water in a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer 12 to 15 minutes until tender and water is absorbed. Remove from heat, let stand covered 5 minutes, then fluff and spread on a baking sheet to cool completely.

In a small bowl, whisk olive oil, lemon juice, oregano, garlic, salt, and pepper. Set aside 10 minutes.

Soak red onion in cold water 5 minutes, drain. Dice cucumber and bell pepper, halve tomatoes, chop herbs. Combine vegetables and herbs in a large bowl.

Add cooled quinoa and half the feta to the bowl. Pour dressing over and toss gently to coat. Taste and adjust salt or lemon, then top with remaining feta and serve.