Chop the king prawns into thirds and mix in a bowl with the crayfish and crab, set aside.
Cook the spinach in a pan with the butter and season with salt and pepper. Once it has cooked leave to cool down and squeeze as much moisture out as you can.
Mix the ricotta, tomato, basil and garlic in a bowl and add to the seafood mixture. Gently stir in until it is evenly coated.
Stir in the spinach and set aside while you make the white sauce.
Melt the butter in a saucepan over a medium heat, then take off the heat. Using a wooden spoon stir in the flour until you have a thick paste. Add the milk and put back onto the heat, using a balloon whisk, keep stirring until your sauce starts to thicken, bring to the boil.
Turn down the heat and add the garlic, salt and pepper and a few gratings of fresh nutmeg, then simmer the sauce for 5 minutes.
Layer the lasagne in a dish starting with a thin layer of the seafood mixture, then the pasta sheets, then a layer of the white sauce. Repeat until all the mixture has gone, finishing with a layer of the white sauce.
Using a food processor whizz up the sourdough until you have breadcrumbs. Sprinkle on top of the last layer of white sauce, then add the parmesan and season the top with salt and pepper. Place in the oven for 35-40 minutes.
Serve with a side salad or garlic sautéed French beans. Enjoy!