Tip the onion and chopped dill into a
bowl with the horseradish and lemon
juice. Stir through the soured cream,
season with a pinch of salt and generous
grinding of pepper, and mix well. Can be
made a day ahead and kept in the fridge.
Arrange the salmon on a board or plate,
scatter with the remaining dill fronds and
serve with a grinding of black pepper and
the sauce alongside.