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The morning before you make the pie, put the butter in the freezer – first wrap it in foil, which will help to keep it cold when you grate it later.
To make the pastry, put the flour in a big bowl and mix in a good pinch of salt and the cheese. Using a coarse cheese grater, grate in the butter – try to hold it as little as possible so it doesn’t warm up too much – and stir it in with a knife as you go. When all the butter is in, give it a final stir to make sure all the strands of butter are coated with flour, then quickly stir in 125ml of very cold water. Splash in a little more if it looks dry, then lightly bring the pastry together with your hands. Wrap in cling film and chill for at least 30 mins.
Meanwhile, bring a big pan of water to the boil, then tip in the potatoes. Boil until just tender, then drain well, slice and allow to cool.
Heat oven to 200C/180C fan/gas 6 with a baking tray inside. On a large sheet of baking parchment dusted with a little flour, roll out the pastry into a rough circle about 30cm wide, until the thickness of 2 x £1 coins. Brush the mustard over the pastry. Arrange the potatoes, asparagus and most of the gruyère in layers in the centre of the base, leaving a border of pastry that’s roughly 5cm all the way around. Season and fold up the pastry edges to enclose the vegetables and cheese, pinching the edges to seal. Whisk together the soured cream and the egg with a little seasoning, then pour into the centre of the tart. Sprinkle over the remaining cheese and slide the tart onto the preheated baking tray. Bake for 30 mins until the pastry is golden and crisp. Eat just warm or at room temperature.