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1
Preheat the oven to 350 degrees F (175 degrees C).
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2
Grease a 9x13 inch baking dish.
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3
Heat the oil in a skillet over medium heat.
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4
Add the onions; cook and stir until tender, 5 to 10 minutes.
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5
Place the potatoes in a separate saucepan and fill with enough water to cover.
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6
Bring to a boil, then cook over medium heat until tender enough to pierce with a fork.
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7
Drain and transfer potatoes to the prepared baking dish.
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8
Add the onions, salmon, haddock, and smoked salmon to the baking dish, tossing to mix.
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9
In the same skillet that the onions were cooked in, melt the butter over medium heat.
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10
Stir in flour so there are no lumps.
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11
Cook and stir for a few minutes, then gradually stir in the milk.
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12
Simmer, stirring frequently, until the sauce begins to thicken.
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13
Season with nutmeg, salt, and pepper; remove from the heat.
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14
Stir in most of the Cheddar cheese until melted, reserving some to sprinkle over the top of the casserole.
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15
Pour the sauce over the contents of the baking dish, and sprinkle with the remaining Cheddar cheese.
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16
Bake in preheated oven until the salmon is cooked through and the cheese on top is toasted, about 30 minutes.
