Preheat oven to 425°F/220°C/Gas 7. Peel and chop the pumpkin, carrots, onions, and potato as directed in the ingredients list.
Place the chopped pumpkin, carrots, and onions onto a large, rimmed baking sheet (or roasting pan) and drizzle with olive oil. Roast for 40 minutes, until soft but not charred.
Meanwhile, place the water and crumbled vegetable stock cubes in a large saucepan over medium heat and bring to a boil. Cook the potato in simmering water, for about 20 minutes until soft.
Place the roasted vegetables into a food processor with the potato and stock. Blend until smooth. Alternatively, you can add the roasted vegetables to the saucepan and use a handheld immersion blender to blend everything together.
Return the soup to the pan over low heat, stirring in the cream, nutmeg, pepper, and salt to taste. Heat gently, then transfer to bowls. Serve immediately, garnished with a swirl of cream, pepitas, grated fresh nutmeg, a drizzle of olive oil, or a sprinkling of fresh chopped herbs if desired.