Rinse quinoa thoroughly, then combine with water and salt in a saucepan. Bring to a boil, reduce heat, cover, and cook for about 12 minutes until liquid is absorbed. Let rest and fluff with a fork.
In a small jar, combine olive oil, lemon juice, garlic, Dijon mustard, salt, and pepper. Shake well to emulsify.
In a large bowl, combine spinach, cooked quinoa, tomatoes, cucumbers, peppers, onion, and avocado.
Drizzle dressing over the salad and gently toss to combine.
Top with feta cheese if desired and serve fresh.
This Quinoa Avocado Salad tastes best when freshly tossed and lightly chilled.