To cook the quinoa: Place rinsed quinoa in a saucepan with water over high heat. Cover pan, bring it to a boil, turn down the heat to low, and let it cook until all the liquid is absorbed, 12 minutes or so. Let it rest for 5-10 minutes before fluffing it with a fork.
Make the Lemon Salad Dressing: Place olive oil, lemon juice, garlic, mustard, salt and pepper in a jar. Put the lid on and give it a shake.
Assemble the salad: Place spinach, cooked quinoa, tomatoes, cucumbers, baby bell peppers, onion, avocado, and cheese in a large salad bowl.
Gently Toss: Drizzle the dressing over the salad. Give it a gentle toss and serve.
