Winter Vegetable Pie with a Parmesan Crust

First of all place the butternut squash, celeriac, carrots and leeks in a steamer (the sweet potato is added later on).

Pour in some boiling water from the kettle, add the freshly grated nutmeg and some salt, then cover and steam the vegetables for 10 minutes.  Now add the chunks of sweet potato and steam for another 10 minutes, or until the thickest parts of the root vegetables feel tender when tested with a skewer. Then tip all the vegetables into a large bowl and allow them to cool.

Meanwhile, make the sauce and the pastry. For the sauce, melt the butter in a smallish saucepan and add the onion. When you've stirred it so that it's nice and buttery, let it cook on the lowest possible heat for about 20 minutes. It's important not to let it colour, so give it a stir from time to time. Now, using a wooden spoon, stir in the flour until smooth, then add the milk a little at a time, switching to a balloon whisk and whisking well after each addition. 

Now season the sauce with nutmeg and salt and pepper, to taste, and let it barely simmer for 5 minutes. After that, stir in the grain mustard, the cheeses and the herbs.  Then leave to cool.

To make the pastry, first sift the flour with a pinch of salt into a large bowl, holding the sieve up high to give it a good airing. Then add the vegetable fat and butter and, using only your fingertips, lightly and gently rub the fat into the flour. When everythign is crumbly, add the parmesan and then sprinkle in some cold water - about 1 tablespoon. Start to mix the pastry with a knife and then finish off with your hands, adding more drops of water until you have a smooth dough that will leave the bowl clean. Then pop the pastry into a polythene bag and let it rest in the fridge for 30 minutes.

When you are ready to bake the pie, preheat the oven to gas mark 7, 425°F (220°C). 

Carefully mix the steamed vegetables with the sauce and pile half of the mixture into the dish. Now sprinkle half of the Gruyere (or other cheese) from the filling ingredients over the top and then repeat with the remaining mixture and cheese. Next, roll the pastry out into a circle on a surface lightly dusted with a little extra flour, and as you roll, give it quarter turns to keep the round shape. Next, cut a 12 inch (30 cm) circle out of this pastry. Roll out the leftover trimmings and cut a ½ in (1 cm) strip to go around the edge of the dish.

Now dampen the edge of the dish with water and place the strip of pastry around the rim, pressing down well. Dampen the top of this strip and then transfer the circle of pastry, rolling it over the rolling pin, to the dish and press it lightly and firmly over the edges to seal. Next, using the blunt edge of a knife, knock up the edges, then flute them using your thumb to push out and your forefinger to pull in again. Then make a hole in the centre of the pastry lid to let the steam out, and brush the surface with beaten egg. 

Now place the dish on the baking sheet and bake it for 25-30 minutes on the centre shelf of the oven, or until the pie is bubbling hot and the pastry is golden brown and crusty.