Coarsely grate 50g of the cheddar. Sift the flour into a mixing bowl, then scoop out 1 tbsp flour and set aside for later. Chop 150g chilled butter off the large block (save the rest for later) and add to the bowl with a pinch of salt. Use your fingertips to rub the butter into the flour till you have a mixture that resembles fine breadcrumbs - if you have a food processor you can use this to blitz them together. Stir in the grated cheddar. Add 6 tbsp cold water to the mix and stir together with a fork to form pastry (or blend in the food processor). Once the pastry has just come together, turn it out onto your work surface, gently knead into a ball, then wrap and chill in the fridge for at least 30 mins to overnight.
Preheat your oven to 200°C/Fan 180°C/Gas 6. Trim and peel the onion then slice it into thin wedges. Pop in a large roasting tin. Peel the sweet potatoes and chop them into small, bite-sized chunks. Add to the tin. Rinse the mushrooms and tear them into small pieces. Add to the tin. Peel the garlic and finely chop it. Add it to the tin with 11/2 tbsp olive oil and a generous pinch of salt and pepper. Toss everything to coat the veg in the oil and seasoning. Make sure it's spread out in one layer across the bottom of the tin, then slide it into the oven and roast for 30 mins till the veg are tender and lightly caramelised.
While the veg roast, tip the cooked chestnuts onto your chopping board and roughly chop them. Coarsely grate the remaining cheddar. Pick the sage leaves off their stems and finely chop them.
When the veg are cooked, take them out of the oven. Stir in the chestnuts, cheddar and sage. If you want to get ahead, you can roast the veg the night before, mix them with the chestnuts, cheddar and sage, then cool and store in the fridge.
When you're ready to bake the galette, make sure the oven is set the oven to 200°C/Fan 180°C/Gas 6. Line a large baking tray with baking paper. Dust your work surface with the reserved 1 tbsp plain flour and roll out one pastry to make a 45cm-wide round. Place this on the baking tray. Spoon the roast veg mix over the pastry, leaving a 4-5cm border around it.
Crack the egg into a bowl and beat it. Brush it all over the pastry, then slide the galette into the oven and bake for 40-45 mins till golden brown and crisp.
While the galette bakes, make the gravy. Fill and boil your kettle. Crumble the vegetable stock cube into measuring jug and stir in 800ml hot water from the kettle to dissolve it.
Chop 25g butter off the large block and melt it in a small pan. Tip in 1 tbsp cornflour and cook, stirring, over a medium heat for 2 mins. Trickle in a little veg stock and stir to combine it with the butter and cornflour mix. Repeat, till you have added all the stock and the gravy mix is smooth and combined. Stir in 2 tbsp cranberry sauce and add a splash of tamari (keep the rest for later). Turn up the heat, bring to the boil, then turn the heat down a little and simmer for 10 mins, stirring occasionally. The gravy will thicken a little. Taste the gravy and add more tamari or cranberry sauce if you think it needs it.
Serve the galette in slices with the gravy and extra spoonfuls of cranberry sauce on the side.