Preheat your oven to 180°C (350°F). Split the squash in half lengthways, place cut-side up onto a baking tray and bake for 45 minutes, or until just soft.
Meanwhile, make the glazed onions. Heat three tablespoons of water in a non-stick saucepan, add the sliced onion and sweat for five minutes. Add the vinegar and sugar, then cook for a further 10 minutes over a low heat, stirring occasionally, until caramelized. Set aside.
Mix the Christmas rice ingredients in a mixing bowl until fully combined, then set aside.
Sauté your mushrooms. Heat two tablespoons of water in a non-stick frying pan over a medium heat. Add the mushrooms, garlic powder and seasoning. Sauté for five minutes.
When the squash is cooked and cooled slightly, scoop out the seeds, then scoop out a 2cm (1in) channel of flesh and mix that into the Christmas rice mixture. Spoon the cranberry sauce into one of the squash halves, followed by the rice. Top with the peppers, onions, mushrooms, sun-dried tomatoes and spinach. Place the other squash half on top, tie together in three places and roast for a further 15 minutes. Carve and serve straight away.