Preheat your oven to 180oc. Cut the peeled butternut squash into chunks and drizzle with the garlic olive oil and rosemary and toss everything together. Roast for about 25 minutes, or until the squash chunks are soft.
Meanwhile prepare the rest of your wellington.
Boil a kettle. Place the spinach in a colander and pour over the just boiled water to wilt it, then set to one side to cool.
Place the oyster mushrooms in a food processor and blitz into small pieces. Set to one side. Repeat with the chestnuts, so they resemble breadcrumbs.
Heat ½ tbsp of the regular olive oil over a medium heat in a non-stick pan and fry the mushrooms off with black pepper for 4-5 minutes until the water they release evaporates. Season and tip into a bowl so they can cool completely.
Heat the remaining oil in a pan and crumble in the chestnuts. Cook for 2-3 minutes until they smell nutty, and then add your gluten free breadcrumbs and cook for a further 1-2 minutes. Take off the heat and stir in the lemon zest.
Once the squash is cooked, tip it into a large bowl with the chestnut bread mixture. Squeeze the wilted spinach to remove any excess water and then chop, and add it to the squash bowl with the lemon juice and crumbled feta. Mix everything together and season with a pinch of salt and black pepper. Leave to cool completely before assembling your wellington, or the pastry will fall apart.
Now assemble your Wellington. Lay a sheet of baking paper onto a large baking tray and lay your puff pastry sheet on top. Spread the mushroom mixture over the top of the pastry sheet leaving a 10cm border on each side.
Next, take handfuls of the squash mixture, squeeze it together and then place in a thick line down the middle of the pastry, leaving space either side so you can fold the edges up to cover the filling.
Make your egg wash by whisking together the egg with the milk. Now, take one side of the baking parchment and lift it over to the middle, so that the pastry folds over the filling. Brush a line of egg wash down the edge of the other side and then bring that up to the middle to seal the filling – the two sides should overlap in the middle.
Now fold each end upwards to enclose the filling and seal with the egg wash.
Carefully roll the wellington over so the seal is on the bottom and brush a little egg wash over the top of the wellington. You can then use a knife to carefully score a criss cross pattern on the top of your pastry.
Bake in the oven for 35-40 minutes until puffed up and golden brown.
Once your wellington is ready, heat your gravy in a pan and pan fry your cavolo nero in the butter.
Serve in slices with the greens and plenty of gravy.