Make the spice paste by combining the ingredients in a blender.
Heat the oil in a large frying pan. Add shallots and curry leaves and cook until softened, about 5 minutes. Stir in the spice paste along with another 3/4 cup water and bring to the boil. Simmer 5 minutes until thickened.
Add the fish and lemon juice. Cook 5 minutes and serve immediately.