Mashed Sweet Potatoes and Carrots with Sweet-and-Sour Walnuts

Name: Mashed Sweet Potatoes and Carrots with Sweet-and-Sour Walnuts
Yield: 8 Servings
Ingredients: 2 pounds red-skinned sweet potatoes (yams); 6 tablespoons butter, divided; 2 pounds carrots, peeled, cut into 1/2-inch-thick slices; 2 tablespoons (or more) water; Pinch of salt; 1/2 cup honey; 1/2 cup Sherry wine vinegar; 1 cup walnuts, lightly toasted