Photograph: Gareth Morgans
Preheat the oven to 180°C, fan 160°C, gas 4. For the fish sauce, over a high heat, cook the mussels in a pan with 50ml of the wine, covered, until the shells open, shaking the pan frequently. Tip into a colander placed over a bowl to collect the liquid. Remove the mussels from the shells and set aside, discarding the shells and any mussels that remain closed.
In a large frying pan, soften the onion in the oil. Add the chilli and garlic, season and fry for another minute. Add the frozen prawns and whole fish fillets, and fry for about 5 minutes, until just cooked. Add the rest of the wine and reduce for a couple of minutes. Add 100ml of the reserved mussel stock, the tomato purée and mussels, and simmer for 5 minutes. Adjust the seasoning, remove from the heat and stir in the parsley. Remove the garlic, flake the fish into chunks and set aside.
Meanwhile, put the milk, onion, bay leaves and grated nutmeg in a saucepan, season and bring to a gentle boil. Melt the butter in a small pan and stir in the flour. Cook for a few minutes over a medium heat, stirring constantly, to make a roux. Remove the bay and onion from the milk, then add the roux, whisking furiously. Cook until it thickens, adjust the seasoning to taste and remove from the heat. Cover the surface with clingfilm to prevent a skin forming.
Spread 2 ladlefuls of the béchamel over the base of a 30 x 20cm ovenproof dish, then a layer of seafood with some of the liquid, a layer of spinach leaves and a layer of lasagne sheets. Repeat to build up about three layers, finishing with a layer of béchamel. Scatter with the parmesan and bake for 40 minutes or until golden.
Get ahead: assemble the lasagne, ready to bake, the day before; chill. Cook for 1 hour. It can also be frozen before baking.