Heat oil in a large saucepan over medium-high heat and once hot add mustard seeds, cooking until they begin to pop. Add onions, garlic and ginger and saute for 2-3 minutes.
Add spices and sweet potato and stir for a few minutes. Add stock to the pot and slowly bring to the boil. Skim any froth that arises to the surface and reduce heat to a simmer, leaving to cook for 30 minutes, stirring often.
When sweet potato is tender, stir in the coconut milk and puree in a food processor until smooth. Season with salt and pepper and serve.