Preheat the oven to 200 degrees Celsius. Cut the pumpkin into uniform pieces and remove the strings and seeds. Peel the ginger and cut it into small pieces. Lightly crush the lime leaves to release their aromas.
Spread the pumpkin pieces on a baking sheet and roast them for thirty minutes in the oven until they are butter-soft. Remove the baking sheet from the oven and transfer the roasted pumpkin to a large soup pot. Add the red curry paste and ginger and heat for a minute to bloom the flavors.
Pour the coconut milk into the pot and add the crushed lime leaves. Heat gently and let it simmer for a few minutes so the flavors blend well. Remove the pot from the heat and discard the lime leaves. Purée the mixture with an immersion blender or in a blender until completely smooth.
Set the pot back on the heat briefly to warm the soup through. Stir well one last time. Divide the hot pumpkin soup into deep bowls or cups and serve immediately.