Use two thirds of the pastry to roll out to fit your flan tin
Filling
Slice the leeks down the middle
Pile into a bowl and wash in plenty of water to remove any grit
Drain well and slice the leeks
Cut the bacon into small pieces and fry until crispy. You may prefer to grill it. If so, grill as whole rashers / pieces and cut up once cooked. Retain all the juices and tip into the cooked leeks as extra flavour.
Tip the drained leeks into the same pan and cook gently until the leeks are well wilted and almost cooked through. If you have grilled your bacon, ensure there is a little water in the pan to cook the leeks in.
Tip into a mixing bowl. If there is quite a lot of liquid in the pan, use a sieve to drain the mix and tip the liquid back into the pan to reduce down.
When sufficiently reduced, tip the bacon and leek mix back into the pan, mix in the remaining reduced liquid, then back into the bowl.
Leave to cool
Add the beaten eggs, double (heavy) cream and mustard if using to the bacon and leek mixture.
Stir very well and pile into pastry case, leaving a tblsp of sauce in the bowl
Roll out the remaining third of pastry and top your pie with it. Tuck in all the edges. Use a pastry brush to paste over the top using the last tblsp of sauce mixture. Cut a steam hole in the top of your pie
Bake
Bake at 180°C Fan / 200°C / 400°F / Gas Mark 6 for about 30 minutes until the filling is cooked through and firm