Preheat the oven to 180C/365F/Gas 4.
Place the milk, bay leaves and garlic into a pan over a medium heat and bring to scalding point (just before boiling point), then remove from heat.
Melt the butter in a separate pan over a low heat, then stir in the flour and cook 2-3 minutes, stirring constantly (don't let it turn brown).
Remove the bay leaves and garlic from the milk, then add a little of the milk to the flour and butter mixture. Stir to make a paste and cook out for 1-2 minutes, stirring constantly.
Gradually add splashes of milk, stirring constantly, until a thick, smooth sauce has formed.
Gradually add the rest of the milk, adding a little at a time and bringing to the boil before adding more milk. Repeat until all the milk has been added. Take off the heat, add the mustard and season well with salt and freshly ground black pepper.
Place a spoonful or two of the sauce into a greased ovenproof dish to just cover the bottom. Cover with a couple of lasagne sheets.
Stir the tarragon, fish, seafood and spinach to the remaining sauce and stir until well combined.
Spoon some of the fish sauce mixture onto the llasagne sheets and top with another layer of lasagne sheets. Repeat this layering of fish sauce mixture and lasagne until all of the sauce and pasta have been used up. The final layer should be lasagne sheets.
Sprinkle the top with the cheddar cheese and parmesan, then transfer to the oven to bake for 40 minutes, or until the fish is cooked and the topping is golden-brown.
Serve at the table in its dish with a bowl of green salad and some garlic bread.