Sauce: Blend thoroughly sour cream, yogurt, lemon juice and sugar. Salad: Peel onions and cut into thin slices. Peel and quarter apples, remove cores and but into thin wedges. Blend onions and apples with sauce. In a dish arrange herring and apple-onion mixture in layers. Cover tightly and marinate in refrigerator for 5 hours. Sprinkle with dill before serving.