Preheat the grill to high. Place the milk in a deep frying pan with the bay leaf, salmon and haddock. Cover the pan with a lid and gently bring to the boil. Add the prawns to the pan right before the milk starts to boil. Once the prawns are pink and the fish is nearly cooked through, turn off the heat. Remove the fish with a slotted spoon and set aside. Pour the milk into a jug and reserve.
Bring a large pot of salted water to the boil, then add the celeriac and boil for 15-20 minutes until tender. Drain well, reserving 100ml of the liquid. Roughly mash the celeriac with a potato masher.
Using the same frying pan you used for the fish, heat half the butter over a medium-low heat and add the onion. Cook, stirring occasionally, for 5-7 minutes, until the onion has softened. Add the garlic and cook for a further 1 minute, then add the remaining butter and the flour. Cook, stirring often, for 1 minute, then start gradually adding the milk from the jug. Increase the heat to medium-high and let the milk come to the boil. Reduce the heat and let it simmer for 2-3 minutes, until thickened. Turn off the heat and stir in the mustard powder, peas and chopped dill, then add the reserved liquid from the celeriac. Season lightly.
Gently flake the fish and place into a 1½ litre ovenproof baking dish along with the prawns. Pour over the sauce. Top the pie with the celeriac mash and sprinkle with the grated cheddar. Place under the grill for 5 minutes, until the cheddar has melted and the topping is lightly crisp and golden.