Preheat the oven to 200°C, 400°F, Gas 6 and put in a baking sheet to warm.
Rub the butter into the flour with the salt until you have coarse breadcrumbs. Add water a little at a time using a round-bladed knife to mix until the mixture just comes together. Form into a ball, cover with clingfilm and refrigerate for 20-30 minutes.
Meanwhile steam the asparagus until just tender to the point of a knife – about 4 minutes. Dry briefly on kitchen towel.
Roll out the pastry and press it gently into a lightly greased flan tin. Prick all over with a fork and bake in the oven on the baking sheet for 20 minutes until pale gold.
Spread the asparagus over the pastry base. Whisk together the eggs, yolk, cream, seasoning and cheese, then pour over the asparagus. Bake in the oven for 25-30 minutes until the filling is just set and lightly gold. Leave to cool.