Wash and trim the spinach leaves; pat them dry with paper towels.
Pit and cube the avocado.
Chop the walnuts.
Combine the lemon juice, olive oil, liquid stevia, and Tamari or Braggs in a large bowl; add the avocado cubes, coating well with the dressing.
Toss the spinach and the walnuts with the avocado and dressing.
Add the mandarin orange sections and toss the spinach salad again.