Heat the oil in a large saucepan over medium-high heat until it shimmers. Add the onion and salt and cook until the onion is softened and translucent, about 5 to 7 minutes.

Stir in the ginger and curry powder and cook until fragrant, about 30 seconds, stirring constantly to ensure the spices do not scorch.

Add the cauliflower and water to the saucepan. Bring the liquid to a simmer, then reduce the heat to medium-low, cover, and cook until the cauliflower is completely tender and easily pierced with a fork, about 15 to 20 minutes.

Working in batches, process the soup in a blender until completely smooth and velvety. Return the pureed soup to a clean saucepan.

Stir in the coconut milk and lime juice. Reheat gently over low heat if needed, then season with additional salt to taste. Serve hot, topped with the sliced scallions.