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Tip the melted butter, lemon zest, mackerel and spring onion into a food processor and blend until smooth. Spoon into a ramekin and smooth the top.
For the topping, melt the 15g butter with the horseradish, then stir in the parsley. Carefully spoon over the pâté to cover it. Press some parsley leaves on top, grind over some black pepper and chill to set the butter. Serve the ramekin of pâté with French bread, butter and extra horseradish sauce.