Scrub the mussels clean. Discard any mussels that don't close when tapped gently.
Put all the ingredients for the mussels in a large saucepan with a splash of water. Place over medium heat with the lid on and cook the mussels until the shells are wide open, and discard any mussels that remain closed.
Remove the mussels from the liquid using a slotted spoon, reserving both the mussels and liqour. Pick the mussel meat from the shells and set to one side.
To make the sauce, put the milk and bay leaf in a sauce pan and warm through for 3–4 minutes. In a new saucepan over medium heat, melt the butter, whisk in the flour then add the hot infused milk little by little until fully incorporated. Stir in the brown crabmeat, tarragon, cayenne, white crabmeat, shrimps and mussels.
To assemble, preheat the oven to 200C/180C Fan/Gas 6.
Blanch the pasta shells in a pan of salted water until just cooked. Refresh in cold water. Mix the breadcrumbs and parsley together in a small bowl.
Spoon the creamy seafood filling into the shells and tightly pack in an ovenproof dish. Pour in the reserved mussel stock, drizzle with olive oil, sprinkle with the crumbs and bake for 25 minutes.
Serve family style in the centre of the table.
