This is a really super recipe from a neighbor in London many years ago. We used to adore it, and I only found it again recently.
Make sure you serve it very chilled.

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Serves 4 to 6

1 cup sour cream
1/2 cup full fat plain yoghurt
pinch of sugar
1 tbsp lemon juice
2 medium onions, thinly sliced
2 tart apples, (Granny Smith) cut into thin slices
1 tbsp chopped fresh dill or 1 tsp dried dill weed
2 x 8 oz jars pickled herring, drained and cut into bite-sized pieces

Combine the sour cream, yoghurt, lemon juice, sugar, sliced onions that have been separated into rings, apple and dill.
Alternate the herring and sour cream mixture in layers in a dish.
Cover and refrigerate for 5 hours at least.
Serve very chilled.