Preheat the oven to 180°C.
Heat milk, bay leaves, and garlic in a pan until just before boiling.
Melt butter in a separate pan and stir in flour for 3 minutes without browning.
Remove aromatics from milk and whisk a little milk into the flour mixture to make a paste.
Gradually whisk in remaining milk until sauce is thick and smooth.
Bring to a boil, then remove from heat and stir in mustard, salt, and pepper.
Spread a small amount of sauce in a greased dish and top with lasagne sheets.
Mix tarragon, fish, seafood, and spinach into the remaining sauce.
Layer the seafood sauce and lasagne sheets until all ingredients are used, ending with pasta.
Top with cheddar and parmesan then bake for 40 minutes until golden.
Serve in the dish with salad and garlic bread.