1. Preheat the oven to 220C/425F/Gas Mark 7. 2. Place butternut squash, celeriac, carrots ( and sweet potato all peeled and cut into 1 inch (2.5 cm) chunks) and leeks trimmed, halved lengthways and cut into 2 inch (5 cm) thick chunks - in a steamer (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. 3. Add 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. Tip all the vegetables into a large bowl and allow them to cool. 4. For the sauce, melt butter in a smallish saucepan and add peeled and finely chopped onion. Stir it from time to time so it won t colour but give nice and buttery result, let it cook on the lowest possible heat for about 20 minutes. 5. Stir in the flour (40g) until smooth using a wooden spoon, add milk a little at a time, switching to a balloon whisk and whisking well after each addition. Season the sauce with grated nutmeg and salt and pepper, to taste, and let it barely simmer for 5 minutes. Stir in the grain mustard, the grated cheeses (Gruyere and parmesan) and finely chopped fresh rosemary and thyme herbs. Then leave to cool. 6. For the pastry, sift the flour (110g plus a little extra for dusting) with a pinch of salt into a large bowl, holding the sieve up high to give it a good airing. Add the lard and butter by the use of fingertips, lightly and gently rub the fat into the grated parmesan and sprinkle in some cold water - about 1 tablespoon. 7. Mix the pastry with a knife and then finish off using hands, add more drops of water until dough become smooth or leave till the bowl clean. Pop the pastry into a polythene bag and let it rest in the fridge for 30 minutes. Preheat the oven on describe above range to bake the pie. 8. Carefully mix the steamed vegetables with the sauce and pile half of the mixture into the dish. Sprinkle half of the Gruyere (or other cheese) from the filling ingredients over the top and then repeat with the remaining mixture and cheese. 9. Roll pastry out into a circle on a surface lightly dusted with a little extra flour, and give it quarter turns to keep the round shape while rolling, cut a 12 inch (30 cm) circle out of this pastry. Roll out the leftover trimmings and cut a