Preheat the oven to 200C/400F/Gas mark 6
Heat the oil in a frying pan and fry the onion and leek until softened. Add the curry paste or powder and cook for a further two minutes. Pour in the coconut milk and slowly bring to the boil.
Add the fish, herbs and spinach, mix until well combined, then remove the pan from the heat. Spoon the mixture into a casserole dish.
Cook the potatoes in a pan of boiling water for seven minutes.
Meanwhile heat the oil in a frying pan and fry the onions for 4-5 minutes, or until softened. Remove the pan from the heat.
Once cool enough to handle, peel the potatoes and grate them into the frying pan with the onions. Stir in the cayenne pepper, salt and freshly ground black pepper. Return the pan to the heat and fry for a further 6-7 minutes.
Spoon the potato mixture onto the fish and bake in the oven for 15 minutes. Preheat the grill to high and continue to cook the fish pie for 5-10 minutes, or until the top is crisp and golden-brown. Serve with a dollop of chilli sauce or mango chutney.