Pull off outer leaves of cauliflower and trim stem. Using paring know, cut around core to remove; thinly slice core and reserve. Cut heaping 1 cup of 1/2-inch florets from head of cauliflower; set aside. Cut remaining cauliflower crosswise into 1/2-inch thick slices.
Melt 3 tablespoons butter or ghee in large saucepan over medium-low heat. Add leek, onion, curry powder and 1 1/2 teaspoons salt; cook, stirring frequently, until leek and onion are softened but not browned, about 7 minutes.
Increase heat to medium-high; add water, sliced core and HALF of the sliced cauliflower; and bring to simmer. Reduce heat to medium-low and simmer gently for 15 minutes.
Add remaining sliced cauliflower, return to simmer and continue to cook until cauliflower is tender, about 15-20 minutes.
Meanwhile, in a medium frying pan, saute remaining sliced onion in 3 tablespoons of butter or ghee, stirring occasionally, until golden brown and crispy. Reserve.
In same pan, add 2 more tablespoons butter and cook reserved cauliflower florets, stirring frequently until florets are golden brown and butter is browned and fragrant, about 6-8 minutes. Reserve florets in a bowl and season to taste with salt and pepper, save remaining butter in the pan for drizzling.
Over a medium heat, toast whole cumin seeds, shaking pan for 1-2 minutes until they pop and crackle.
Puree soup in batches in a blender or food processor. Add water if necessary if soup is too thick.
Garnish each bowl with reserved florets, toasted onions and cumin seeds. Drizzle each bowl with melted browned butter.