Combine Salad Ingredients. In a large bowl, combine cooked and cooled quinoa, apples, celery, spinach leaves, cranberries, walnuts, (and cheese, if using).
Make Dressing. In a small bowl, whisk together mustard, honey, vinegar, olive oil, salt and pepper. (Dressing will keep 1 week in the refrigerator.)
Dress Salad. Add half of the dressing to the salad and stir to combine.
Taste & Adjust. Then, taste and add additional dressing, as desired. (If I'm making the salad in advance, I like to add the rest of the dressing right before serving, since the salad will continue to absorb dressing in the fridge.)
Serve & Store. Enjoy right away or chill in an airtight container in the fridge 3-4 days.