How to Make It
Preheat the oven to 375º. On a large rimmed baking sheet, toss the cauliflower with the cumin seeds, curry powder and 3 tablespoons of the oil. Season with salt and pepper and roast for about 25 minutes, turning occasionally, until the cauliflower is just tender.
In a large saucepan, heat the remaining 1 tablespoon of oil. Add the onion and cook over moderate heat, stirring occasionally, until softened but not browned, about 5 minutes. Add the roasted cauliflower, butter, bay leaf and water and bring to a simmer. Cook over moderate heat until the liquid is reduced and the cauliflower is very soft, about 15 minutes. Pick out and discard the bay leaf.
In a blender, puree the soup in two batches until very smooth. Return the soup to the saucepan and stir in the milk. Rewarm it over moderate heat, adding more water for a thinner consistency, if desired. Season the soup with salt and pepper and serve hot.