1. 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 chunks. Add to the tin. Rinse the mushrooms and tear them into small pieces. Add to the tin. Peel the garlic cloves and finely chop them. Add to the tin with 1½ tbsp oil and a generous pinch of salt and pepper. Toss to coat the veg in the oil and seasoning. Make sure it’s spread out in one layer across the bottom of the tin. Roast for 30 mins till the veg are tender and caramelised.
2. While the veg roast, tip the cooked chestnuts onto your chopping board and roughly chop them. Coarsely grate the Cheddar. Pick the sage leaves off their stems and finely chop.
3. 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.
4. When you’re ready to bake the galette, make sure the oven is set to 200°C/Fan 180°C/Gas 6. Line a large baking tray with baking paper. Unwrap the pastry and gently knead the 2 blocks together to make 1 round. Dust your work surface with a little flour from your cupboard and roll out the pastry to make a 45cm-wide round. If you don't have any flour, you can roll the pastry out between 2 sheets of baking paper.
5. Place the pastry round on the baking tray. Spoon the roast veg mix over the pastry, leaving a 4-5cm border around it. Fold the pastry up over the filling, leaving a gap in the middle.
6. Crack the egg into a bowl and beat it. Brush it all over the pastry to glaze it, then slide the galette into the oven and bake for 40-45 mins till golden brown and crisp
7. While the galette bakes, make the gravy. Fill and boil your kettle. Crumble the vegetable stock cube into a measuring jug and stir in 800ml hot water from the kettle to dissolve it.
8. Melt 25g butter in a small pan. Tip in 1 tbsp cornflour and cook, stirring, over a medium heat for 2 mins. Trickle in a little of the vegetable 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. Stir in 2 tbsp cranberry sauce and add a splash of the balsamic vinegar (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 balsamic vinegar or cranberry sauce, if you think it needs it.
9. Serve the galette in slices with the gravy and extra spoonfuls of cranberry sauce on the side.
10. Love Your Leftovers
Any leftover galette will keep in the fridge for 2-3 days. It’s best served cold in slices, alongside a crunchy green salad and some extra roast potatoes.