In large heavy saucepan or Dutch oven, heat oil over medium heat; cook onions, garlic, celery, ginger, fresh and ground coriander, cumin, salt, pepper, lemon rind, turmeric and hot pepper flakes, stirring until aromatic and onions are softened, 5 to 8 minutes.
Stir in squash and tomato; pour in stock and bring to boil. Reduce heat to low; cover and simmer until squash is tender, about 20 minutes.
In blender or food processor, purée soup, in batches, until smooth. Return to clean saucepan; reheat until steaming. Stir in coconut milk; heat through. Serve sprinkled with coriander leaves.