Make the pastry
Put the flour, icing sugar, salt, mustard and chilli in a bowl and rub in the butter until all the lumps disappear and you’re left with a crumbly mixture.
Stir in the grated cheese. Mix together the egg with a teaspoon of cold water, then stir through and mix to form a smooth, sticky dough. Make the dough softer than is rollable, adding a little more water if you think it needs it, then scoop it out in one lump on to a sheet of non-stick paper.
Smooth out the dough into a block, then wrap the paper around it and pop it in the fridge until firm. Baking the tart pastry
To begin with, sprinkle a fine layer of flour both under and over the dough, then start to roll it out. As soon as the first cracks appear around the edges, stop rolling and slide a spatula or palette knife under the pastry. Dust more flour under and over the dough, and quickly roll out further, patching together any frayed edges with your fingers.
Baking the pastry
Butter and flour an 8” loose bottom tart tin. Lift the pastry into the base and gently push into place, patching up where needed. Line with parchment and weight down with rice or lenils and pre-bake at 180C (160C fan-assisted)/350F/gas mark 4 for about 20 minutes, then carefully remove the baking beans and paper, lower the heat to 170C (150C fan-assisted)/335F/gas mark 3 and bake for a further 15-20 minutes, until barely golden on the base.
Make the tart filling and bake
First prepare the asparagus, snap the woody bits off the base of the spears, steam them for about four minutes, or until al dente. remove and cool in cold water, drain well and dry on a clean tea-towel.
Cut the asparagus spears into short lengths and set the tops aside for later. Put roughly half the rest of the asparagus pieces in a food processor and blitz to a puree.
Pour the double cream into a jug, beat in the eggs and horseradish, and mix in the blitzed asparagus purée
and grated cheese, season well with salt.
Arrange the remaining asparagus pieces, including the tops, on the base of the pastry, then pour in the egg mixture. Mix together the Parmesan, breadcrumbs and herbs and sprinkle over the top.
Bake for about 35-40 minutes, until set and golden on top, but still a little wobbly in the middle. Leave to cool slightly, or completely, before serving – if you make this in advance, keep it covered, but at room temperature to prevent the pastry going soggy.
PASTRY
275g plain flour, plus extra for rolling
2 teaspoons icing sugar
1 level teaspoon sea salt
1 teaspoon English mustard powder
¼ teaspoon chilli or cayenne powder
150g unsalted butter, cut into small chunks
30g grated Parmesan
1 large egg, beaten
1 dessertspoon chopped chives
Ice-cold water
FILLING
300g asparagus
284ml double cream
2 eggs, beaten
1 rounded teaspoon creamed horseradish
50g gruyere or nutty cheddar, or vegetarian alternative, finely grated
1 heaped teaspoon grated Parmesan
1 tablespoon breadcrumbs
1 heaped tablespoon chopped herbs e.g. chives, parsley