For the spice paste, place all of the spice paste ingredients into a food processor and blend until smooth. (You can use a pestle and mortar for this, too.)
For the curry, heat the vegetable oil in a wok and gently fry the onion and garlic for 4-5 minutes, or until softened. Add the sweet potato, curry powder and cumin and continue to fry for further 5-10 minutes, until the sweet potato has softened, then add the fish and two tablespoons of the spice paste. Stir to coat the fish in the spices, then add the coconut milk and bring to a gentle simmer.
Stir in the baby spinach leaves, lime juice and season with salt, to taste. Cook for another 2-3 minutes, then stir in the coriander.