Preheat the oven to 190°C, fan 170°C, gas 5.
Take the squash and cut a crisscross pattern in the flesh. Place cut side up in a roasting dish, drizzle with the olive oil and dot with the butter. Roast for 30 mins or until almost tender.
Mix the chestnuts with the leaves from the thyme and the cranberries.
Spoon into the cavity of the squash, dot with a knob of butter and roast for 15 mins until the squash is tender. Serve either on its own as a vegetarian dish, or as a side dish to a roast dinner.