Preheat oven to 190 degrees C / Gas 5. Pour 250ml of water into 23x33cm or similar sized baking dish.
Place the butternut squash into the prepared baking dish, cut side down.
Bake in preheated oven until squash is easily pierced with a fork, about 45 minutes.
Meanwhile, place the rice and remaining 250ml water into a saucepan. Bring to the boil, uncovered, over medium-high heat. Reduce heat to low, cover and simmer until water is absorbed and rice is fluffy, about 20 minutes. Remove from the heat and fluff with a fork.
Heat the olive oil in a large stock pot over medium-high heat. Stir in the onion and cook until tender and transparent, about 5 minutes. Mix in the turkey sausage; cook until crumbly and evenly browned. Drain any excess fat. Stir in the cooked rice and corn.
Scoop out cooked squash and place in a liquidiser or food processor. Discard squash peels. Pour chicken stock into the liquidiser or food processor with the squash. Blend until smooth, about 1 minute.
Stir squash mixture into the sausage mixture until well blended. Season with pepper and salt to taste. If desired, stir in the double cream. Simmer soup over medium heat until heated through, about 15 minutes, but do not boil.