Layered marinated herring salad

INGREDIENTS
1 sweet onion, thinly sliced
250ml (9fl oz) soured cream
120ml (4fl oz) plain yogurt
1 tbsp fresh lemon juice
1⁄4 tsp caster sugar
2 tart dessert apples, peeled, cored, and thinly sliced
2 pickled dill cucumbers, sliced or chopped
salt and freshly ground black pepper
300g (10oz) marinated herring fillets, drained
2 cooked potatoes, diced (optional)
1 cooked beetroot, sliced (optional)
1 tbsp chopped dill, to garnish

METHOD
Put the onion in a bowl, cover with cold water, and leave to soak for 15 minutes. Drain well, then toss with the soured cream, yogurt, lemon juice, and sugar. Stir in the apple and pickles, and season to taste with salt and pepper.
Place half the herring in a serving dish and top with the potatoes and the beetroot, if using. Cover with half the soured cream sauce. Layer the remaining herring, potatoes, and beetroot over the sauce, then add the remaining sauce.
Cover the dish tightly with cling film and refrigerate. Sprinkle with dill just before serving.