3 tbsp olive oil
200g/8oz chestnut mushrooms, halved if large
2 tsp light muscovado sugar
100g/4oz blue cheese, such as Stilton, or vegetarian blue cheese
250g/9oz puff pastry, defrost if frozen
Heat oven to 200C/180C fan/gas 6. Peel the onions and cut each into 6 wedges through the root. Heat the oil in a large pan, add onions, then gently fry for 20 mins until softened and lightly coloured.
Add the mushrooms, sugar, salt and pepper and give it a good stir. gently cook, stirring now and then for a further 5 mins. stir in the walnuts. line the base of a 20-23cm sandwich cake tin (not loose-based) with baking parchment. spoon over the onion mixture and press it down lightly. Crumble the cheese over.
Roll out the pastry and trim to a round, about 5cm larger than the tin. Put the pastry over the filling and tuck in the ends. Bake for 35-40 mins until the pastry is crisp and golden. Cool for 5 mins in the tin, then turn out onto a flat plate and cut into wedges. serve with a green salad.