Prepare the Mackerel: Start by removing the skin from the mackerel fillets. Flake the flesh into a food processor, making sure to remove any bones you encounter.
Blend the Pâté Ingredients: Add the grated horseradish (or horseradish sauce), Dijon mustard, a generous amount of freshly ground black pepper, crème fraîche, and lemon juice to the food processor with the mackerel. Blend until the mixture is smooth.
Butter: Add the chopped unsalted butter to the mixture in the food processor. Blend again until the butter is fully incorporated and the pâté has a creamy, smooth texture.
Taste and Adjust: Taste the pâté and adjust the seasoning with more black pepper or lemon juice if needed.
Serve: Serve the smoked mackerel pâté chilled, garnished with chopped spring onions. Accompany with slices of sourdough bread for spreading.
