Preheat the oven to 200°C/400°F/Gas mark 6.
Melt the butter in a saucepan over a low heat. Add the flour and mustard powder, and stir until smooth. Cook gently for 2 minutes without colouring.
Gradually add the milk, whisking until smooth. Bring to the boil and simmer for 2 minutes. Remove from the heat and reserve. Cover the surface of the sauce with cling film to prevent a skin forming.
Heat the oil in a large frying pan over a low heat. Add the onion, garlic and thyme, and sweat for 5 minutes until softened. Add the mushrooms and cook for a further 5 minutes until softened. Stir in the wine, and boil rapidly until nearly evaporated. Stir in the tomatoes. Bring to the boil, reduce the heat slightly and simmer, covered, for 15 minutes. Season with salt and pepper.
Lightly oil a lasagne dish. Spoon half of the tomato sauce over the bottom of the dish, and top with half of the fish and scallops. Layer half of the lasagne over this, and cover with half of the white sauce. Dot with half of the mozzarella. Repeat the layers, finishing with the mozzarella.
Bake in the oven for 30–40 minutes until bubbling, golden and cooked through. Remove from the oven and leave to stand for 10 minutes before serving hot.