Put the butter in a large sauté pan or large, deep, frying pan over low-medium heat.

When melted, stir in the sliced leeks and season with a little salt and pepper.

Gently cook, stirring often, until they're softening without browning (10 -12 min), adding the garlic for the final 2 minutes.

Sprinkle the flour over the vegetables then cook for 1 minute, stirring.

Take the saucepan off the heat and pour in approximately 100ml of the milk. Stir well so it's combined with the other ingredients with no lumps.

Gradually add the rest of the milk and the chicken stock, stirring constantly and scraping up any browned bits from the bottom of the pan.

Put back on the heat, turned up to medium, and continue stirring until the sauce bubbles and thickens.

Turn the heat to low and cook for 2-3 minutes, stirring.

Fold the chicken and ham into the sauce along with a generous grind of pepper plus the fresh herbs. Taste and add more seasoning if needed.

Transfer the filling to a bowl or suitable container, cover and put in the fridge until cold.

If using a food processor

Put the flour, salt and butter into the food processor and whizz until the mixture looks like very fine breadcrumbs.

With the motor still running add iced water, 1 tbsp at a time, until the mixture starts coming together.

If making by hand

Stir the flour and salt together in a bowl. Rub in the butter so that the mixture resembles fine breadcrumbs.

Add 1 tbsp of cold water and stir it in with a round bladed knife. Continue adding tablespoons of water and stirring until the mixture starts coming together.

Turn out onto a lightly floured surface and briefly knead into a smooth ball of dough.

Wrap in cling film or foil and chill in the fridge for at least 30 minutes.

Line two or three baking trays with baking paper.

Preheat the oven to 180°C / 160° Fan / Gas 4

Divide the dough into 8 equal pieces (or 6 for larger pasties), using scales for accuracy. Shape each piece into a ball.

On a lightly floured surface, roll out the first ball into a circle approximately 20 cm in diameter (or 27 cm if you're making 6 larger pasties): use a suitably sized plate, bowl, or saucepan lid as a template to cut around and get a neat circle.

Put ⅛ (or ⅙) of the filling on one half* of the circle, leaving a 2 cm bare edge. Brush the edge with a little of the beaten egg then bring over one side of the dough to cover the filling and create a semi-circular pasty.

* to accurately find the halfway mark, gently fold the dough over, make a light crease then open it up again.

Press the edge down well to seal, then repeat with the tines of a fork (dipped in flour if they stick) to create a pattern or use your fingers to crimp the edge.

Transfer to a baking tray.

Repeat the process with the remaining balls of dough and filling.

Brush the pasties with beaten egg yolk, put in the oven, and bake for 25 minutes.

Take the trays out of the oven and brush the pasties with another layer of beaten egg.

Turn the heat up to 200°C / 180° Fan / Gas 6 and return the pasties to the oven.

Cook for a further 5 - 7 minutes or until the pastry is cooked, crisp and golden brown.

If not serving straight away, cool then store in the fridge for 2 days or freeze.

Reheat at 180°C / 160° Fan / Gas 4 until piping hot (approx. 20-25 min) or heat in the microwave for 2 minutes and put in the oven for 5 minutes to crisp up.