Preheat the oven to Gas Mark 7, 220°C, fan 200°C. Line a baking sheet with nonstick baking paper. Mix the custard, vanilla and ground almonds together.
On a lightly floured surface, roll the pastry to a rectangle, about 23 x 30cm (9 x 12in), then lay it on the lined baking sheet. Spread the custard mix over the pastry, leaving just under 2.5cm (1in) space around the edges. Scatter the nectarine slices over the custard, then roll up the bare pastry edges to make a slight rim.
Sprinkle over the icing sugar, then the flaked almonds. Bake for 15-20 minutes until the nectarines are tender and the pastry is crisp.
Sieve the conserve then brush it carefully over the fruit and pastry edges. Serve hot with cream or crème fraîche.