Butternut, mushroom and chestnut brioche-crust wellington

Method

1. For the brioche, stir in the yeast, sugar flour and salt together in the bowl of a stand mixer (or a large mixing bowl). Make a well in the centre and pour in the milk and 2 beaten eggs, then use the dough hook attachment on the mixer to knead the dough for 8-10 minutes until smooth and stretchy. At this point, add the 125g butter, a piece at a time, mixing well to incorporate each piece before adding the next. You should end up with a very soft, glossy dough. Cover and chill overnight and return to room temperature for 1 hour before rolling out (step 5).
2. To make the filling, heat the oven to 200ºC/180ºC fan/gas 6. Toss the squash with oil in a large roasting tin, season with salt, then roast for 30 minutes until soft and caramelised. Set aside
3. Melt 25g of the butter in a large sauté pan or frying pan. Add the onion and leek with some salt and cook over a medium heat for 6-7 minutes until softened but not coloured. Remove to a large bowl with a slotted spoon, add the remaining butter to the pan and turn up the heat, then add the fresh mushrooms with more salt and fry, without stirring, for 2-3 minutes. Give them a stir, then leave to fry for another 2-3 minutes until softened and coloured. Turn down the heat and add the garlic, chestnuts and dried mushrooms, stirring to coat in the butter. Cook for a few minutes more until fragrant. Tip into the bowl that contains the onion and leek mixture, then leave to cool.
4. Once cool, mix in the herbs, nuts, breadcrumbs, flour, cheese, eggs, squash and a generous amount of seasoning until combined. Put in the fridge while you roll out the brioche.
5. Punch down the brioche dough to knock out any air, then roll out on a lightly floured surface to a rough 35cm square the thickness of a £1 coin. Lift onto a sheet of non-stick baking paper. Tip the filling onto the bottom third of the brioche, leaving a 3-4cm border at the bottom and sides, then shape into a fat sausage. Bring the dough over the top of the filling, lift the bottom border up to touch the filling, then cover it with the top of the brioche sheet. Tuck the edges under the wellington on the 2 open sides to seal the filling inside the brioche, then carefully use the baking paper to help transfer the wellington to a baking tray. Brush with beaten egg, then prove in the fridge for 40 minutes. Heat the oven to 180ºC/160ºC fan/gas 4.
6. Once the wellington is slightly puffed and firm, glaze all over again. Then, using the tip of a very sharp knife or scalpel, score a pattern into the top, making sure you don’t cut completely through the dough. Bake for 40 minutes until the brioche is deep golden and crisp. Remove from the oven, leave to cool for 5 minutes, then slice and serve with traditional roast accompaniments or a bitter leaf salad, if you’d prefer.