Prepare the quinoa according to package instructions. Use a larger pot than necessary so that you have room to add spinach once the quinoa is finished cooking. (note: the key to a great quinoa is rinsing it very well and/or soaking it before cooking!)
While the quinoa is cooking, sauté the red onion over medium heat in a couple teaspoons of olive oil, until it is completely soft and browned, about 10 to 15 minutes. Set aside.
In a mixing bowl, whisk together the olive oil, vinegar, and lemon juice.
Once the quinoa is finished cooking, remove it from the heat and add the baby spinach to the pot, putting the cover back on to help steam the spinach. Allow the spinach to steam and soften about 5 minutes and then mix it in to the quinoa.
Add the quinoa, spinach, sauteed onions, sun-dried tomatoes, olives, and feta cheese
Pour the oil/vinegar/lemon dressing and sea salt over the salad and toss everything together until coated. Taste the salad for flavor, and add more sea salt and/or lemon juice to taste.