Heat oil in a medium saucepan over medium heat and cook onion, stirring, until softened, about 5 minutes. Add garlic, cilantro stems, and coriander and cook, stirring, until fragrant, about 2 minutes. Add squash and 2 1/2 cups water and bring to a boil. Reduce heat and simmer, covered, until squash is just tender, about 15 minutes. Let cool slightly, then season with salt.
Puree soup in batches until smooth. (If too thick, thin with a little water.) Adjust seasoning if necessary and let cool slightly. Garnish with cilantro leaves and serve with lime.