Pre-heat a sauté pan over medium heat. Add the oil and sauté the onion and leek until the onion starts to soften – about 4 to 5 minutes. Add the garlic, thyme and potatoes and continue to cook for another 3 minutes or so.

Sprinkle the flour over the vegetables and stir to coat evenly until no traces of white flour can be seen. Cook for another 1 to 2 minutes.

Combine the stock, heavy cream and mustard, stirring to dissolve the mustard, and pour this into the pan. Simmer very gently for another 4 to 5 minutes.

Remove the pan from the heat and stir in the ham, parsley and lemon zest. Season to taste with salt and freshly ground black pepper (careful with the salt because ham is salty) and transfer the mixture to a pie dish to cool.

When the filling is cool. roll out the pastry and top the pie dish, tucking a layer of extra pastry under the rim of the pastry and pinching the dough to create a pretty edge. Chill the pie in the refrigerator for at least 20 minutes while you pre-heat the oven to 425˚F.

Brush the pastry with egg wash and transfer the pie to the oven for 30 to 40 minutes, or until nicely browned on top and bubbling.