For dressing, in a medium bowl beat or whisk together the yogurt, lemon juice, sugar, and salad oil. Season to taste with salt and white pepper. Cover and chill in the refrigerator until serving time.
In a salad bowl combine greens, orange slices, and onion rings. Drizzle with dressing. Sprinkle with walnut halves. Serve at once. Makes 4 side-dish servings.