1) Line a large baking tray with non-stick baking paper. Place the puff pastry sheet on a lightly floured surface and cut into 4 equal pieces. Transfer to the baking tray, well spaced apart.
2)Make a border around the outside of each pastry square by scoring a line with a sharp knife about 1cm in from the edge to create an inner square. With a fork, prick the dough inside the small square. Cover and refrigerate for at least 30 minutes and up to 1 day.
3) Preheat the oven to 200C/Gas 6.
4) In the centre of each piece of pastry, overlap slices of apple and nectarine, keeping within the scored border. Brush the fruit with a little melted butter and sprinkle each tart with 1 tsp of sugar.
5) Bake the tarts until the pastry is golden and apples are tender, about 20 minutes. Remove tarts from the oven and brush each one with 1 tsp of honey. Transfer tarts to a cooling rack.
6) Meanwhile in a small bowl, lightly whisk the mascarpone with the orange liqueur.
7) Serve tarts topped with a dollop of the flavoured mascarpone and drizzled with extra honey if desired.