Remove root and leave 1-inch stem on beets; scrub; wrap each one in foil. Place beets on baking pan. Roast for 45 minutes to an hour or until tender. When cool enough to handle, rub off skins and slice into wedges.
Prepare vinaigrette by combining olive oil, vinegar, orange juice, 1 teaspoon orange rind, salt and pepper in a small bowl; whisk to combine.
Cut avocado in half, remove pit; scoop avocado from skin with large spoon; slice each half into 6 slices.
Divide salad greens among 6 plates; top each with beets, oranges, onion and avocado.
Drizzle vinaigrette over salad.
Garnish with remaining orange zest and toasted walnuts.