In a large, heavy pot, melt the butter. Add the onion and sliced ginger and cook over moderate heat, stirring occasionally, until the onion is softened, 7 minutes. Add the curry paste and cook, stirring, until fragrant, 2 minutes. Add the squash and water and bring to a boil. Cover partially and simmer over low heat until soft, 25 minutes. Add the coconut milk, lime leaves and lemongrass, cover partially and simmer for 30 minutes longer. Discard the lime leaves and lemongrass.