Preheat the oven to 450 degrees. Spread the cauliflower on a baking sheet, drizzle with oil, and sprinkle with salt. Toss to combine. Roast until edges are brown, about 25 minutes.
In a medium stockpot, melt the butter over medium heat. Add the onions, and cook, stirring frequently, until the onions begin to soften, about 4 minutes. Add the curry powder and cook, for 1 minute more. Add the water and chicken broth. Add half of the cauliflower to the pot, reserving the rest. Cover, and bring to a boil. Uncover, lower the heat, and simmer until all the vegetables are soft, about 5 minutes.
Working in batches, puree the soup in a blender until smooth. If necessary, return to the pot to heat through. Transfer to a large serving bowl. Taste and adjust for seasoning. Serve hot garnished with parsley and reserved cauliflower.