Pre-heat the oven to 180C/gas mark 5. Lightly grease an 18-20cm wide, 3cm deep, straight-sided flan ring or rectangular tart tin with a removable bottom (if your tin doesn’t have a separate base, then it’s best to use a buttered, bottomless flan ring on a buttered tray).
Roll out the pastry on a floured surface until it’s about 3mm thick, then lay it into the tin or flan ring. The best way to do this is to roll the pastry around the rolling pin, then unroll it over the tin and ease the pastry into the tin with your hands. Press the pastry firmly into the corners of the tin and patch up any holes by pinching the pastry together, or by patching in some of the excess pastry. It may look fragile, but it’s more forgiving than it seems.
Roll the rolling pin across the top of the tin to trim off the excess pastry, then neaten up the edges by going round and pinching them with your thumb and forefinger. Leave to rest in the fridge for 20-30 minutes.
Line the tart tin with greaseproof paper, fill with baking beans and bake blind for about 10-15 minutes, or until it begins to colour lightly. Remove from the oven, leave to cool and remove beans.
Meanwhile gently cook the leeks in the butter for 4-5 minutes with a lid on, stirring every so often until soft and without colouring then drain in a colander. Cook the asparagus in boiling salted water for 3-4 minutes until tender then drain and leave to cool. Chop the asparagus into rough ¾ cm chunks and arrange in the tart case with the leeks, ham and cheese. Whisk the eggs, cream and milk together and season. Pour into the tart tin and bake for 20-30 minutes until the filling has just set.
You may need to amend the egg, butter and cream quantities if your tart tin is larger or deeper. You can serve this either warm or cold