Make the Pastry
You can use shop-bought shortcrust pastry, but I prefer to make my own. I almost always use hot water pastry for savoury pies, because it's very easy to make, and very easy to work with. Chop the lard into chunks (so they melt faster) and place in a pan with the water. Heat the pan until the lard has fully melted into the water.
In a large bowl, stir the salt into the flour, then add the water and lard. Mix thoroughly, tip out of the bowl and wrap in plastic film. Put the pastry in the fridge or freezer to cool to room temperature.
Make the filling
Top and tail the leeks, slice thinly and boil for about 10 minutes. Peel and finely chop the onion and sauté in vegetable oil until soft but not browned. Cut the chicken and bacon into small chunks, about 1cm (half an inch).
Make a White Sauce
Melt the butter in a medium-sized saucepan. Stir in the flour and mix thoroughly. Add the milk, stir well and heat until it thickens. Add the stock, pepper and oregano, followed by the drained leeks, onion, chicken and bacon. Simmer for 10-15 minutes until the chicken is cooked. Remove from heat and cool to room temperature.
It's very important that both your pastry and the filling are cool before you assemble the pie, otherwise you'll get a soggy bottom.
Assemble the Pie
Preheat your oven to 230°C (446°F). Grease a 24cm (9 inch) diameter pie dish with butter. Take a little less than half of the pastry and roll it out on a floured surface until you have a disc about 3-4mm (bit less than a quarter of an inch) thick. Place the upturned pie dish onto the pastry and cut around the edge. This is the lid: set it to one side, then roll out the remaining pastry. Gently place the pastry into the tin, pressing it well into the edges. Trim off any overhanging pastry.
Spoon the filling into the pie. Moisten the rim with cold water, then place the lid on it. Press all around the edge with your thumbs to form a good seal. If you like, you can do some fancy crimping.
Pierce the lid with a skewer - this allows the steam to escape during cooking and stops the pie from bursting. Glaze the top with beaten egg and put the pie in the oven to bake for about 40 minutes. Remember all ovens are different, so have a peek at the pie before the end of cooking to make sure it's not burning.
Serve with potatoes and one or two token vegetables.