Pre-heat the oven to 200°C, 180°C fan, 400°F, Gas Mark 6.
Unroll the whole (375g) piece of puff pastry on a lightly floured surface and slice it widthways, slightly off-centre, to give two pieces; one larger than the other. The larger piece should be about 20 x 30 cm. Chill or freeze the smaller piece for another time.
Score a 2cm border all the way around the rectangle with a sharp knife and prick the centre all over with a fork. Transfer to a baking tray lined with nonstick baking paper and chill it for a few minutes.
Meanwhile, blanch the asparagus spears in a large saucepan of boiling, salted water for 2 minutes. Refresh in cold water and drain well.
Arrange the asparagus spears across the centre of the pastry.
Bake in the oven for 10 minutes, until the pastry border has risen. Meanwhile, combine the crème fraîche with the milk, egg, nutmeg and half the cheese in a jug. Season with salt and pepper.
Quickly remove the tart from the oven and pour the crème fraîche mixture over the asparagus, inside the pastry border. Scatter with the reserved cheese and return to the oven for 10 minutes, until golden brown.
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