Herring Salad with Sour Cream (Heringstopf mit Saurer Sahne)

2 cups diced boiled potatoes
1 1/2 cups marinated herring filets, cut into small pieces
1 cup chopped celery with leaves
1 tablespoon minced chives
1 tablespoon minced parsley
1 cup finely cut onions
2 grated tart apples

Sauce:
1/4 cup plain yogurt
1/2 cup sour cream
1/4 cup mayonnaise
3/4 teaspoon paprika
2 tablespoons lemon juice
1 teaspoon Dijon mustard
1/2 teaspoon salt
1/4 teaspoon white pepper

Combine sauce ingredients. Add other ingredients to a large bowl and pour sauce over it, tossing well. Marinate in refrigerator for at least 1 hour.

Serve on inner tightly cupped lettuce leaves. (Other variations include using hardcooked eggs and pickles; garnish plate with chopped beet slices.)