Time: 1 hour 20 minutes
Preheat the oven to 200C/Gas 8
Place the potatoes into a large pan of water and bring to the boil. Cook for about 15 minutes until the potatoes are really soft and just starting to fall apart. Remove from the heat, drain and then return to the pan with a lid on to steam whilst you make the filling.
Drizzle the oil into a medium saucepan and add the onion. Cook over a gentle heat for about 5 minutes until the onion is soft. Add the butter, melt slightly and then add the flour and garlic. Keep stirring to make a roux-the flour and butter will have mixed together to form a thick paste. Don’t allow it to brown.
Add the milk a little at a time and keep stirring vigorously to avoid lumps.
When all the milk has been added, keep stirring until the sauce just starts to boil. Turn off the heat and stir in the Crème Fraiche and lemon zest. Season well with salt and ground pepper.
Place the spinach, parsley, fish and prawns into a large, deep, ovenproof dish. Pour over the sauce and mix together well.
Add the hot milk to the cooked potatoes and mash until smooth. Stir in most of the cheese, spring onions and season with salt and pepper. Carefully spoon the mash over the fish filling, sprinkle over the remaining cheese and place in the oven for 45-50 minutes, until golden brown and bubbling.
Serve with buttered peas.