Preheat the oven to 200ºC/400ºF/gas 6. Slice the squash lengthways into wedges and add to a large roasting tray with a good splash of olive oil, the chilli and cinnamon. Bash the coriander seeds in a pestle and mortar until fine, then add the rosemary leaves and bash again for 1 to 2 minutes to release its flavour. Scatter over the squash and toss together so that each piece of squash is well coated with the seasoning. Make sure all the squash is skin-side down, then cover with tin foil and bake in the hot oven for around 45 minutes or until soft. Allow to cool, then tear into bite-sized chunks.
Meanwhile, heat a saucepan over a medium heat, then add a splash of olive oil and the onions. Season well with salt and pepper and cook gently, stirring occasionally, until softened and lightly browned. Add the sage and crumbled chestnuts to the pan for the last few minutes of cooking.
While that’s happening, toast the bread on a hot griddle pan or in a toaster and rub well with one of the cloves of garlic. Tear into small chunks, and once the onions are done, add the toast to the pan. Turn the heat off, stir everything together, taste, then season and grate in the zest of the lemon.
Add the butter to a frying pan on a medium heat and when melted, add the mushrooms and a chopped clove of garlic. Fry until soft and quite dry. Squeeze in a little lemon juice, tip into a food processor and whiz until smooth.
Bring a large pan of salted water to the boil, add the spinach and cook until soft. Drain in a colander, pressing lightly to get rid of excess moisture, then place to one side.
Slice the remaining garlic clove and add to a frying pan with a splash of olive oil. Fry until golden. Add the pine nuts, sultanas and spinach and fry everything together until warmed through. Season well with salt and pepper and turn off the heat.
Now assemble your Wellington. Roll out the puff pastry on a sheet of baking parchment until it’s about 30cm x 40cm, then spread the mushroom mixture all over it. In a large bowl, lightly toss together the spinach, squash and onion-bread mixture, then spoon it in a thick line down the middle of the pastry. Leave a space free at either side so you can roll the pastry around the filling.
To do this, hold one side of the baking parchment and lift it, with the pastry, towards the centre of the Wellington so it starts to cover the filling. Peel the baking parchment back, leaving the pastry in place, then do the same with the other side. The pastry should overlap in the middle. Beat the egg with the milk and brush it over the pastry join to seal the join. Fold up the ends so the filling doesn’t leak out, then carefully roll the Wellington onto a baking sheet, with the seal underneath. Brush all over with the egg mix.
Bake for 45 minutes until puffed up, golden brown and hot through. Serve carved into thick chunks – it’s fantastic served with veggie gravy!