Preheat the oven to gas 6, 200°C, fan 180°C. Roll out the pastry large enough to line a deep, 23cm (9in) round tart tin with a removable base. Trim the edges, then prick the base with a fork and chill for at least 30 minutes.
Meanwhile, arrange the rhubarb in a large ovenproof dish with 120g (4oz) of the sugar and the split vanilla pod. Cover loosely with foil and bake for 20-25 minutes, until the rhubarb is just tender but still holds its shape. Drain the liquid and set the rhubarb and vanilla pod aside.
Cover the tart base with a piece of nonstick baking paper and fill with baking beans or rice. Blind bake for 15 minutes, then remove the paper and beans and bake for a further 5 minutes, until cooked but still pale. Immediately brush the base with egg white to seal.
Reduce the oven temperature to gas 3, 170°C, fan 150°C. Combine the remaining sugar and cornflour in a bowl and gradually whisk in the eggs and crème fraîche. Scrape out the vanilla seeds from the reserved pod and whisk in.
Arrange the drained rhubarb in the pastry case and pour over the custard mixture. Bake in the oven for 30 minutes, or until just set in the middle. Allow to cool in the tin before serving in slices.