Preheat the oven to 200°C/400°F/gas 6.

Melt the butter in a deep saucepan. Add the leek and gently sauté for 5–6 minutes until soft. Add the vinegar, then stir in the flour and cook over the heat until blended. Add the milk, bring to the boil, then stir until thickened. Add the lemon juice, spinach, dill and 50g (2oz) Parmesan. Stir over the heat until the spinach has wilted. Season with salt and black pepper.

Put a third of the salmon, prawns and broccoli into the base of a 21 x 16 x 7cm (8¼ x 6¼ x 2¾in) dish. Pour over a third of the sauce. Place two sheets of lasagne on top. Repeat with more of the fish mixture, sauce and the remaining lasagne sheets, then finish with a layer of the fish mixture and sauce. Sprinkle with the 30g (1oz) Parmesan, then place in the oven for 30 minutes. Leave to stand for 5 minutes before serving.