Skin the mackerel and break the fillets up into small pieces. Add the soft cheese, lemon zest and horseradish sauce and mix well together with a fork.

Season with salt and freshly ground black pepper, to taste, and add lemon juice, to taste. Stir in most of the herbs then cover and chill until ready to serve.

Sprinkle with a little cayenne, if using, and remaining herbs and serve with warm, buttered toast.