In a kettle cook the onion in the butter over moderately low heat, stirring, until it is softened, add the broccoli, the mushrooms, the potato, peeled and cut into 1 inch pieces, the curry powder, and the broth, and simmer the mixture, covered, for 15 minutes. In a blender or food processor puree the soup in small batches, transferring it as it is pureed to a tureen, and stir in the lime juice Divide the soup among 6 to 8 soup bowls garnish each serving with a lime slice, and serve it with a dollop of the sour cream. (Alternatively, the soup may be chilled and served cold.) Serves 6 to 8.