Spread walnuts in a single layer in a shallow baking pan and toast until golden (about 5-7 minutes). Let cool slightly.
In large bowl, stir together yogurt, lemon juice, cinnamon and honey. Core, quarter and thinly slice apples. Add to dressing, mixing gently to coat.
Line salad plates or a large platter with leaf lettuce. Mound apple slices over lettuce, spooning any remaining dressing over apples. Sprinkle with roasted walnuts. Serve immediately.