Step 1:
Heat the oil in a large frying pan or saucepan over medium heat. Add the chicken and cook, stirring occasionally, until browned all over but not fully cooked through, about 6–8 minutes. Remove chicken to a plate and leave the oil in the pan.

Step 2:
Add the butter to the pan. Fry the leeks gently on medium-low heat for 8–10 minutes until softened but not browned. Stir in the garlic and cook for 1 more minute. Pour in the white wine and simmer for a few minutes until the alcohol evaporates and the liquid reduces slightly.

Step 3:
Sprinkle the flour over the leek mixture and stir until it forms a thick paste with no lumps. Gradually add the chicken stock, stirring constantly to combine and thicken the sauce, about 5 minutes. Pour in the milk, bring to a gentle simmer, and cook for 1 minute, stirring continuously to prevent lumps.

Add the bay leaves, thyme sprigs, browned chicken, and chopped ham. Season generously with freshly ground black pepper and add salt sparingly (the ham and stock will add saltiness). Pour the filling into a large pie dish (around 30 x 20cm).

Note: The filling can be refrigerated for up to 2 days or frozen for up to 3 months. If frozen, cool completely first and defrost overnight in the fridge before proceeding.

Step 4:
Preheat the oven to 200°C (180°C fan) / gas mark 6. Dust your work surface with flour and roll out the puff pastry to about ½ cm thick. Cut a pastry disc large enough to cover the pie dish with a slight overhang.

Brush the rim of the pie dish lightly with beaten egg. Lay the pastry over the filling and press the edges to crimp and seal. Use any leftover pastry to decorate the top if you like, sticking decorations in place with a little beaten egg.

Brush the entire pastry surface with the remaining egg wash. Bake for 45 minutes to 1 hour, until the pastry is golden and the filling is bubbling hot.

Storage:
The assembled pie can be chilled (unbaked) for up to 2 days or frozen for up to 3 months in a freezer-safe ovenproof dish. If baking from frozen, cook for 1 to 1½ hours until piping hot throughout.