Melt butter in a large saucepot over medium heat and cook onion 3-4 minutes to soften, but not brown. Add sweet potato, ginger, coconut milk and 3 cups of stock. Simmer 25 minutes, until sweet potatoes are tender. Purée until smooth with a hand blender, add remaining stock if soup is too thick, stir in lime juice and strain. Return to heat and season to taste.
Ladle into bowls and top with coriander leaves and a swirl of yogurt.