Preheat the oven to 200°C.
Fry the onion and leek in oil until softened.
Stir in curry powder and cook for two minutes.
Pour in coconut milk and bring to a boil.
Mix in the fish, herbs, and spinach, then transfer to a casserole dish.
Boil potatoes in water for seven minutes.
Fry the second onion in oil for five minutes until softened.
Peel and grate the potatoes into the frying pan with the onions.
Season with cayenne, salt, and pepper, and fry for seven minutes.
Top the fish with the potato mixture and bake for 15 minutes.
Grill on high for 10 minutes until the top is crisp.
Serve with sweet chilli sauce or mango chutney.