Soak the herring over night; remove the milch and mash fine. Cut off the head, skin and bone; chop the herring; add chopped apples, pickles, potatoes, olives and capers. Put in the salad bowl; then add the yolk of a hard-boiled egg to the mashed milch, mustard, 1 teaspoonful of sugar mixed with ¼ cup of vinegar and a little lemon-juice, salt and pepper. Pour the sauce over the salad and garnish with olives and sliced lemon.