Poach the fish in milk and bay leaves until just cooked. Remove, flake into bite-sized pieces, and set aside.
Cook any other seafood you're using at this point.
Melt butter in a saucepan, add flour to make a roux.
Gradually add milk and fish stock while whisking to avoid lumps.
Cook over medium-low heat until thickened. Season with salt, pepper, and herbs/spices.
Preheat the oven to 200°C/180°C fan/gas mark 6.
Place the fish filling in a baking dish. Pour the sauce over it.
Top with a layer of creamy mashed potatoes and create a textured surface with a fork.
Bake for 30-35 minutes until golden on top and heated through.