For Melba toast, toast the bread in a toaster, remove and cut off the crusts then slice through the side edge to make two thin slices. Grill, untoasted side up, until golden, then cut in half diagonally.

Peel, core and cube one of the apples and cook until slightly softened in the butter. Set aside to cool. Soak the leaf gelatine in cold water for a few minutes to soften. Squeeze excess water from the sheet before adding it to 100ml just boiled water, and allow to cool.

Remove the skin and blitz the mackerel fillets and cooked apple together. Add the cream, horseradish sauce, 1-2 tablespoons of lemon juice (to taste), pinch of cayenne, fresh dill and cooled gelatine. Blitz until combined, taste, then season with lots of black pepper.

Whisk two egg whites to soft peaks, gently fold in to fish mixture.

Lightly oil six ramekins or moulds with an unscented oil and pour mackerel mousse into each ramekin. Fill to the top. Allow to set for four hours, or overnight, in the fridge.

Cut the apples for the garnish into wafer thin discs. Dip the slices in lemon juice and press between two sheets of clingfilm so they do not discolour.

To serve, release mackerel mousses from moulds (briefly dip each ramekin base in a bowl of boiling water) and turn out. Garnish with baby watercress and serve chilled with the Melba toast.