Use a rimmed roasting pan to spread out the vegetables and an immersion blender to make the soup smooth in the pot.
Method
Preheat the convection oven to 400°F. Cut the cleaned pumpkin, leek whites, onion, garlic and carrot into similarly sized large chunks and place them in a rimmed roasting pan.
Add the table salt, cumin, turmeric, black pepper and olive oil. Toss so the vegetables are evenly coated.
Roast for 30 minutes, until the pumpkin is tender and the vegetables are lightly browned on top.
The roasted pumpkin is tender, and the vegetables are lightly browned rather than charred.
Transfer the roasted vegetables to a soup pot. Use some of the measured hot water to loosen the flavorful browned bits in the roasting pan with a spatula. Pour the juices and loosened bits into the pot, then add the remaining measured water.
Bring to a boil and cook for 5 minutes. Remove the pot from the heat and blend the soup until completely smooth, following your blender's instructions.
After blending, the soup is smooth with no visible vegetable chunks.
Return the soup to low heat. Add the cold butter and stir until incorporated, then remove from the heat. Ladle into bowls and serve.
Cooking guidance
Roasting the vegetables brings lightly browned edges and a deeper sweetness to this pumpkin soup. Cumin and turmeric add warm flavor, while cold butter stirred in at the end gives the soup a smooth finish.
Weigh the pumpkin after peeling and removing the seeds. Use only the white parts of the thin leeks, and cut the vegetables into similarly sized chunks so they roast evenly.
Cut the vegetables to a similar size for even roasting.
The measured water includes the portion used to loosen the roasting pan juices.
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