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1
Place potatoes into a large pot; add 1 bay leaf, salt, and pepper. Cover potatoes with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and return potatoes to pot. Mash potatoes with 8 ounces milk and 1/4 cup butter until smooth.
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2
Preheat oven to 200 degrees F (95 degrees C). Spread olive oil into the bottom of a 12-inch square baking dish.
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3
Combine remaining 20 ounces milk and 1 bay leaf in a large, deep saucepan and bring to a simmer. Cook haddock, salmon, and smoked fish, working in batches if needed, until fish flakes easily with a fork, about 5 minutes. Remove fish from milk and separate into chunks on a work surface. Arrange fish and shrimp in the bottom of the prepared baking dish, reserving milk.
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4
Pour the milk used to cook the fish into a measuring cup, adding more if needed to equal 2 1/2 cups.
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5
Heat remaining butter in the same large saucepan over low heat; slowly whisk flour into melted butter until smooth. Add the 2 1/2 cups milk, leek, parsley, scallions, and mustard; simmer until thickened, about 5 minutes. Pour milk mixture over fish and shrimp and top with the mashed potatoes, spreading evenly.
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6
Bake in the preheated oven until bubbling around the corners and lightly browned, 30 to 45 minutes.
