Creamed herring (śledź w śmietanie)

Years back, when she heard that I was doing a Polish Christmas Eve dinner, my grandmother sent me this recipe that she found in a Polish-American newspaper. The recipe called for a jar of marinated herring and I made it like that the first few years. Last year, though, I decided to try a different herring -- one packed in oil -- and decided I liked it much better because the herring is almost like butter. You serve the herring with rye bread. Here's what you need:

1 pkg. of herring in oil (or a jar of marinated non-creamed herring, in which case you discard all the onions and spices)
a few slices of onion, sliced wafer thin
1/2 apple, finely diced
3/4 cup sour cream
1/4 cup mayonnaise
1-2 t brown mustard (something spicy like Polish or Düsseldorf style, but not sweet like Bavarian)
juice of 1/2 a lemon
dash of pepper
paprika (for dusting)
lettuce (like Romaine or Boston)

Fork blend the sour cream, mayonnaise, mustard, lemon juice and pepper, then taste and maybe add a little more mustard if you like it spicier. Lay the lettuce leaves on a serving plate then cut the herring up into smaller pieces and lay them on the bed of lettuce. Sprinkle the sliced onions and diced apple among the herring. Right before serving, pour the cream sauce over the herring, onions and apple and dust it with paprika.