Preheat the oven to 180°C. Combine salmon and trevalla in a large frying pan with onion, bay leaf, and 2 1/2 cups (625ml) of milk. Simmer on high heat until just cooked through, about 5 minutes. Remove from heat and transfer fish to a plate to cool slightly. Flake the fish into large pieces and place in a large bowl.
Pass milk mixture through a fine sieve into a jug and discard solids. Melt half of the butter in a saucepan until foamy. Stir in flour and cook for 2 minutes until bubbly. Slowly pour in milk mixture, stirring constantly. Cook for 5 minutes until sauce thickens. Remove from heat, stir in half of the cream, season with salt and pepper. Combine with prawns, scallops, salmon, chives, and dill. Gently mix before transferring to a 6-cup ovenproof dish.
Cook the potatoes in a pot of boiling salted water until tender. Drain and mix with butter, milk, and cream in the same pot. Mash until smooth, then spoon over the fish mixture.
Place in the oven until the dish is a beautiful golden brown and fully heated. Serve right away.