1 tsp freshly grated horseradish (or 2 tsp horseradish sauce)
½ lemon, juice only
spring onions, chopped
2 tsp Dijon mustard
freshly ground black pepper
100g/2½oz unsalted butter, chopped
2 tbsp crème fraîche
2 whole smoked mackerel (or 4 fillets)
sourdough bread, cut into slices