Preheat oven to 200 degrees C. Wash beetroot and place in a baking dish with 60ml water. Cover and roast for 1 hour or until tender. Remove from oven and set aside to cool.
Whisk together the apple juice, vinegar, oil, salt, pepper, mustard and celery salt.
Peel and slice the beetroot and combine with the dressing. Refrigerate for at least 30 minutes.
Divide the salad leaves among four salad plates. Drain beetroot, and reserve dressing. Decoratively arrange overlapping layers of beetroot, onion, apple and avocado on the salad greens. Drizzle with dressing. Garnish with toasted walnuts and serve immediately.