1In 4- to 6-quart Dutch oven, place steamer basket; add 1/2 inch water (water should not touch bottom of basket). Place sweet potatoes in basket; cover tightly and heat to boiling. Reduce heat to low; steam covered 16 to 17 minutes or until tender. Plunge potatoes into ice water until cold; drain.
2Meanwhile, sprinkle walnuts in ungreased heavy skillet. Cook over medium heat 5 to 7 minutes, stirring frequently until nuts begin to brown, then stirring constantly until golden brown.
3In large bowl, sprinkle apples with sugar and lime juice; toss gently to coat. Stir in sweet potatoes, raisins, salt and 3 tablespoons of the walnuts.
4In small bowl, stir together yogurt and mayonnaise. Pour over sweet potato mixture; toss gently to coat. Sprinkle with remaining 1 tablespoon walnuts. Cover; refrigerate at least 2 hours or up to 12 hours.