Preheat your oven to 200°C/Fan 180°C/Gas 6. Peel the butternut squash, then slice it in half and use a spoon to scoop out the seeds (see our tip on how to make the most of them, overleaf). Chop the squash into bite-sized chunks and scatter them into a large roasting tin. Season with salt and pepper and drizzle over 1 tbsp olive oil, then toss to coat. Slide the squash into the oven and roast for 45-50 mins, turning once or twice, till tender and caramelised.
Scatter the cooked chestnuts into a second roasting tin. Thickly slice the mushrooms. Peel the red onions and slice them into thin wedges. Peel and thinly slice the garlic. Pick the thyme leaves off their sprigs. Add the mushrooms, red onions, garlic and thyme to the chestnuts. Pour over 1 tbsp olive oil and add a pinch of salt and pepper. Toss the veg to coat them in the oil and seasoning, then slide onto the oven shelf below the butternut squash. Roast for 20-25 mins, till tender.
Meanwhile, make the pastry. Tip 300g of the flour into a large mixing bowl. Grate in 100g of the cheddar. Measure out 100g of the cold butter (keep the rest for your other recipes) and cut it into small cubes. Add to the bowl. Use your fingertips to rub the flour, cheese and butter together till the mixture resembles breadcrumbs. Separate 1 egg yolk from the white (keep the white in a small bowl for later). Add the egg yolk and 6 tbsp cold water to the mixing bowl and stir together to make a dough. Add 1-2 tbsp more water if the pastry still looks a little dry. Wrap the pastry in greaseproof paper or a beeswax food wrap and pop it in the fridge for 30 mins.
When the butternut squash is tender and caramelised, scoop half of it into a large bowl and mash it with a fork. Rinse the kale leaves and strip them off their stalks. Finely shred the kale leaves and add them to the mashed squash. Grate the remaining cheddar into the bowl and scoop in half the jar of crème fraîche. Add 1 tbsp wholegrain mustard and a little salt and pepper. Crack in the remaining egg and stir well.
Fold the remaining squash pieces into the bowl. Add the roast mushrooms, onions and chestnuts to the bowl and fold together. This is your galette filling.
Lay the chilled pastry on a large sheet of greaseproof paper. Roll the pastry out into a large round, approximately 35cm across, using the reserved 50g flour to dust your rolling pin if needed. Slide the rolled pastry and greaseproof paper onto a large baking tray.
Scoop the veg and cheese filling into the centre of the pastry, and spread out, leaving a gap of approx 5cm around the edges. Fold the pastry up around the filling to create free standing sides. Brush the pastry sides all over with the reserved egg white.
Slide the galette into the oven and bake for 40-50 mins, till the pastry is golden brown and very crisp. Let the galette cool slightly before serving with the Sticky Onion & Mushroom Gravy and spoonfuls of cranberry sauce.