Heat the oil and a small knob of butter in a pan. Add the onion and garlic and cook for about 15 minutes until well softened.
Add the leeks and continue to cook for 2 minutes, then tip into a colander and leave to drain.
In the same pan, melt 75g (3oz) of the butter and then stir in the flour to make a paste. Cook for 3-4 minutes, without colouring, then slowly ladle in the milk, stirring constantly with a wooden spoon, until you have a glossy sauce. Stir in the mustard, lemon juice and season to taste.
Preheat the oven to 180C (350F), Gas Mark 4. To assemble the pie, place the leeks in the bottom of a 1.6 litre pie dish that is about 22cm in diameter) and top with a third of the sauce. Follow with the ham and another third of the sauce. Finally scatter over the Cheddar and top with the remaining sauce, making sure it is covering the filling completely.
To make the filo crust, melt the butter in a small pan, then add the garlic. Heat gently for 5 minutes to allow the garlic to infuse. Lay one sheet of filo on a chopping board and brush all over with the garlic butter, then sprinkle with some chives and cracked black pepper. Place another sheet of filo on top at a slight angle and repeat. Continue in this way until you have used up all of the pastry and most of the garlic butter.
You should have created a jagged edge pastry circle. Lay this on top of the pie dish, tucking in the edges of the pastry into the dish. Brush with a final layer of the garlic butter, then sprinkle with salt and bake for 35-40 minutes until the pastry is crisp and golden.
Serve straight to the table with some boiled baby potatoes or a side salad.