1. Heat nuts in a small nonstick skillet over medium-low heat, stirring often, 8 to 10 minutes or until lightly toasted and fragrant.
2. Arrange sweet potatoes in a steamer basket or stainless-steel colander in a 4- to 6-qt. Dutch oven over boiling water. Cover and steam 16 to 17 minutes (up to 30 minutes) or until potatoes are fork tender. Plunge potatoes into ice water to stop the cooking process; drain.
3. Place apples in large bowl. Sprinkle with lime juice and no-calorie sweetener; toss gently to coat. Stir in sweet potatoes, raisins, salt, and 3 Tbsp. walnuts.
4. Stir together yogurt and mayonnaise. Add yogurt mixture to sweet potato mixture, tossing gently to coat. Sprinkle with remaining 1 Tbsp. walnuts. Cover and chill 2 to 12 hours.
Note: For testing purposes only, we used Splenda No Calorie Sweetener.