Steam sweet potatoes until just tender, about 6 to 7 minutes. Do not overcook as you want the sweet potato pieces to hold its shape. Plunge into cold water. Drain.
In a large bowl, toss apples and lemon juice. Add celery, raisins, cranberries, walnuts, and sweet potatoes. Sprinkle nutmeg over fruits and vegetables. Add mayonnaise and toss to coat.
Chill for at least an hour before serving.
To serve, place baby greens on a plate. Spoon salad onto baby greens.