You will also need a round baking dish with a diameter of 9 inches and at least 2 inches deep.

Place the butternut squash, celeriac, carrots and leeks in a steamer. Pour over some boiling water and add some freshly grated nutmeg and a little salt, then cover and steam for 10 mins. Then add the sweet potato and steam for 20 mins or util the thickest parts of the veg are tender when tested. Tip all the veg into a large bowl and allow to cool.

Meanwhile, make the sauce and the pastry. For the sauce, in a small saucepan melt the butter and add the onion, and when you have stirred it well and the onion is all covered in butter, let it cook on the lowest heat for about 20 mins - it is not to brown the onion, so give a stir now and then. Next, using a wooden spoon stir in the flour until smooth, then add a little milk at a time and stir - switch to a ballon whisk - and whisk well after each adition of milk. Season to taste with nutmeg, salt and pepper, then let it barely simmer for 5 mins. Remove from the heat and stir in the mustard, cheeses and herbs. Leave to cool.

To make the pastry, first sift the flour with a pinch of salt into a large bowl, holding the seive up high to get in air. Then add the fats, and using your fingertips gently rub in the fat to the flour until it resembles breadcrumbs. Then add the parmesan, mix, then sprinkle in some cold water - about 1 tablespoon. Using a knife start to mix the pastry together, finishing with your hands and adding a little more water until the dough comes together enough to leave the bowl clean. Wrap the dough in plastic wrap and let it rest in the fridge for 30 mins.

(Up to this point, you can prepare the pie in advance, then assemble it nearer to the time you will bake it).

When you are ready to bake the pie, preheat the oven to 425F. Carefully mix the steamed veggies with the sauce and pile half of it into the pie dish. Now sprinkle in half the Gruyere (or Emmental or other cheese) from the filling ingredients over the top and repeat with the remaining mixture of veggies and cheese. Next, roll out the pastry enough to cut out a 12inch circle from it, then reroll the trimmings to make 1/2 wide strips that will go around the edge of the dish.

Dampen the edges of the pie dish with water and place on the strip of pastry around the rim, pressing down well. Dampen the strip and the transfer the circle of pastry over the top of the dish, pressing down lightly but firmly at the edges. Next, using your fingers flute the edges of the pastry, then make a whole in the center with a knife to let out the steam. Finally brush the pastry all over with the beaten egg. (Don't do this last bit until you are just about to bake it, but you can assemble the pie a couple of hours in advance if you need - just keep it cool and covered so the pastry does not dry out).

Now place the pie dish onto a baking sheet and bake it in the center of the oven for 25-30 mins or until the pie is bubbling hot and golden brown and crusty.