Method

Start by making the pastry. In a large bowl sift in the flour with the salt. Add the butter and lard, rubbing in with your finger tips to make rough “breadcrumbs”. Stir in the cheese, thyme leaves and pepper. In a small bowl whisk the egg yolks together with 1 or 2 tablespoons of water, add to the bowl of “breadcrumb” mixture and mix in with a dinner knife to form larger pieces of smooth dough that will eventually come together as one piece. Add a little more water if very dry, but take care not to add too much. Loosely shape into a rectangle, wrap in cling film and leave to chill in the fridge for 30 minutes.
Preheat the oven to 180c/160c fan). Very lightly dust a clean work surface and a rolling pin with a little flour. Take a 12 cm x 35cm rectangular loose bottomed tart tin or a 23cm loose bottomed round tart tin. When the pastry has rested, unwrap and place on the floured work surface.
Carefully roll out the pastry, moving it in quarter turns after each couple of rolls until it is the thickness of a pound coin and you can line the tart tin with overhang.
Roll the pastry loosely up around the rolling pin and drape it over the tart tin. Carefully press the pastry into the tart tin and up the sides. Trim off the overhang and use it to patch up any holes. Prick the base of the tart case with a fork and place the tin back in to the fridge or freezer to chill for a further 20-30 minutes. Meanwhile, place a pan of water on to boil and fill a large bowl half full with cold water and ice. Trim the asparagus, add to the pan of boiling water and cook for 1 minute. Quickly drain the asparagus, plunge into the bowl of cold water and place to one side. This will help the asparagus retain its colour and stop it from drying out too much when baked.
Place a large frying pan on a medium heat, to begin making the filling. Add the butter and oil to the pan and allow to slowly melt. Trim, slice and wash the leeks and add to the pan. Slowly cook the leeks on a low heat until soft, golden and sweet (around 15 minutes). Spoon out of the pan and place on a plate to cool. Wipe out the pan and place back on the heat. Slice the bacon and add to the pan, fry for a few minutes until crisp and add to the plate with the leeks to cool.
When the pastry has had its second round of chilling, remove from the fridge and place on a baking tray. Line the case with a piece of greaseproof paper and fill the case with baking beans or rice. Place in the oven and bake for 15 minutes.
When time is up carefully remove the baking beans and piece of greaseproof paper. Return the case to the oven and bake for a further 5 minutes until lightly golden.
Remove the tart case from the oven and reduce the temperature down to 160c/140c (fan).
In a large mixing bowl, whisk together the double cream, crème fraîche, eggs and extra yolk, salt, pepper and mustard to make a smooth “custard”, then crumble in the goat’s cheese, if desired, and season well. Spoon the leeks and bacon into the tart case, then carefully pour the mixture into the prepared tart case. Remove the asparagus from the water and pat dry, then arrange the asparagus spears over the tart and push them down a little, until halfway submerged.
Place the tart back into the oven and bake for 25-30 minutes until golden and set. When the tart is ready, remove from the oven and allow to cool a little. Serve warm or cold.