Preheat the oven to 200°C.
Boil the apples, 50g sugar, butter, and 3 tablespoons of water in a saucepan.
Simmer covered for 3 to 5 minutes until the apples soften.
Uncover and cook for 3 minutes until the mixture becomes a purée.
Adjust sweetness with sugar and cool the purée.
Place one pastry sheet on a greased tray and cover with marzipan slices leaving a 2cm border.
Spread the apple purée over the marzipan and brush the border with egg yolk.
Fold the second pastry sheet and score it at 1cm intervals along the fold.
Unfold the pastry, place it over the filling, and crimp the edges to seal.
Trim the edges, brush with egg, sprinkle with demerara sugar, and bake for 30 minutes at 200°C.
Cool slightly and serve with clotted cream.