First, make the topping. Combine the dried fruit and orange juice in a pan, and heat gently over a low heat for 5-10 mins until all the juice has been absorbed and the fruit is plump. Set aside and leave to cool.
Heat the oven to 200C/180C fan/gas 6. Toss the squash with 1 tbsp oil in a roasting tin. Season and roast for 25-30 mins until soft, stirring after 20 mins to prevent the squash from burning.
Meanwhile, oil a 24cm bundt or savarin tin. Line the base with baking parchment – this is easier if you cut out smaller rectangular pieces of parchment and overlap them in the base. Heat another 1 tbsp oil in a frying pan over a low heat and cook the onions until tender, about 8 mins. Add the mushrooms, season and cook for another 5-8 mins, or until the mushrooms are soft and any liquid they’ve released has evaporated. Remove from the heat and leave to cool slightly. Mix the flaxseed with 8 tbsp hot water, and leave until the mixture is gel-like.
Rub a little oil over the whole sage leaves and arrange in the base of the tin. Press the fruit topping over the leaves so the base is fully covered.
Combine the roasted squash, cooled onion and mushroom mixture, flaxseed, nutmeg, breadcrumbs, chopped sage leaves, rosemary, lentils and chestnuts. Season well, then pack into the tin over the fruit. Cover with foil. The unbaked wreath will keep, covered in the fridge, for two days.
With the oven still at 200C/180C fan/gas 6, bake the wreath for 45 mins, then remove the foil and bake for another 15 mins. Leave to cool in the tin for a few minutes, then run a cutlery knife around the inner and outer edge to loosen it. Place a plate over the tin, invert, then lift off the tin. Remove the baking parchment and serve.