Begin by preheating the oven to 400°F. Cut open the Japanese pumpkin and remove the seeds and pulp. Chop into smaller, 2-inch cubes.
Arrange evenly on a baking sheet, drizzle with olive oil and add a sprinkle of sea salt. Bake for about 40 minutes.
While the pumpkin is roasting, finely chop the white onion and cook for about 8 minutes on low heat in the unsalted butter.
When the onion is translucent, transfer off the heat.
In a food processor, blend 2 cups of heavy cream for about 5 minutes or until it has been whipped and looks lighter.
Add the cooked white onion, garlic powder, sea salt and fresh rosemary. You can also use dried rosemary, about 2 tablespoons. Blend to incorporate.
When the pumpkin has roasted, allow it to cool a bit, then peel the skins off. Don’t worry if some of the skin stays attached. Throw the peeled pumpkin into the food processor and blend until completely combined and creamy. This may take some time.
Transfer the pumpkin soup mixture to a pot on medium heat. Add 1/2 cup to 1 cup of water or broth to thin the soup out and heat until desired warmness.