In a large saucepan, cook the onion & minced ginger in the butter over low heat until onion is soft. Add squash, broth, 2 cups water & the garlic. Bring mixture to a boil & simmer covered for 15-20 minutes or until squash is tender. Puree mixture in batches in a blender or food processor & then return to the pan. Stir in lime juice, season with salt & pepper and reheat soup.
In a small skillet, heat the oil over moderately high heat until it is hot & fry julienned ginger. Float a lime slice in soup & sprinkle with fried ginger.