Preheat oven to 400 degrees F (200 degrees C). Wash beets, and place in a baking dish with 1/4 cup water. Cover and bake for 1 hour or until tender. Remove from oven and set aside to cool.
Whisk together the apple cider, vinegar, oil, salt, pepper, mustard and celery seed.
Peel and slice the beets, combine with the vinaigrette and refrigerate at least half an hour.
Divide the greens among four salad plates. Drain beets, and reserve dressing. Decoratively arrange overlapping layers of beet, onion, apple, and avocado on the greens. Drizzle with reserved dressing. Sprinkle on the nuts, and serve at once.
