Put fish in a saucepan and cover with cold water. Add split lemongrass, ginger, half the onion, split chilli, and fish sauce.
Simmer, covered, for 10 minutes. Carefully lift out fish and reserve. Strain stock and set aside. Using a mortar and pestle, pound garlic, grated ginger, shrimp paste, turmeric, sliced lemongrass and chopped chilli to a paste.
In a pan, heat the oil and c