Preheat the oven to 180 degrees.

First make the pastry. Place the flour in a food processor and blitz with the S blade for a few seconds to ensure the flour has no lumps. Cube the cold butter and add it to the flour. Pulse the mixture a few times until it resembles breadcrumbs. Add the egg and a teaspoon of water. Pulse for a few seconds at a time until the mix begins to clump together to form a dough. Gather it all together and gently pat into a flat disc. Cover in cling film. Place it in the fridge for the gluten to rest so your pastry will be light and buttery.

Wash and finely chop the leeks. Use all of the green parts, too. Melt the butter in a heavy-based pan, add the leeks and sauté until beginning to melt and becoming soft. Add the crushed garlic and stir; continue to cook for another few minutes.

Mix the cornflour with a few tablespoons of milk to form a smooth paste, add the Dijon mustard. Add this paste to the leeks and stir. Gradually pour in the rest of the milk until everything is well combined. Leave to bubble for a few minutes over a medium heat. Season to taste with salt and pepper. Pour the pie filling into a deep, oven-proof dish. I used a 30cm enamel pie dish.

Roll the pastry out on a lightly floured surface. Cut the pastry to form a lid to fit your chosen dish. Gently lay the pastry over the pie filling. Crimp the edges and make some cuts in the top to let steam escape. Glaze the pie with the beaten egg and scatter with chia or poppy seeds. Bake for 40 minutes until golden brown around the edges. Serve with just-cooked green peas that have butter and shredded mint leaves folded through.