Line a baking sheet with baking parchment, and unroll the pastry on top. Prick all over with a fork, then spread the mincemeat over the pastry so it’s covered in a thin layer, leaving a 1cm border around the edges. Chill for 20 mins.
Roll the marzipan out on a work surface dusted with icing sugar to a roughly 30 x 21cm rectangle that’s about 2mm thick. Drape it over the rolling pin and lay over the pastry, on top of the mincemeat layer. It should fit neatly in the middle of the pastry, within the border. Chill for another 20 mins.
Heat the oven to 200C/180C fan/gas 6 with a baking sheet inside. Peel and core the apples, but keep them whole. Using a mandoline or sharp knife, thinly slice the apples crosswise, so you end up with thin apple rings. Arrange the apple rings over the marzipan in an overlapping tight spiral shape that goes from the outside into the centre, or in neat overlapping lines. Dot over the butter and sprinkle with the sugar.
Bake for 35-40 mins until the apples are golden and caramelised, and the pastry is cooked through. Serve in small slices with crème fraîche. Will keep, covered in the fridge, for up to three days.