This is a simple fish pie with leeks or onions, topped with cheesy mashed potatoes. I prefer to use a variety of seafood here, but you can stick with one if you'd like. If you do, use the total weight of the fish and seafood below.
Make mashed potatoes. Put the potatoes in plenty of water and add a hefty pinch of salt. Bring to a boil and cook until the potatoes are soft, about 20 minutes. Use this time to chop and prep things.
While the potatoes are cooking, bring the 1 cup each of milk and heavy cream to the steaming point in a medium pot, and drop the fish in to poach. Let this cook gently for 10 minutes, then lift out the fish. When it's cool enough to handle, flake it. Reserve the milk-cream mixture. Flake out the smoked fish, too.
By now your potatoes should be ready. Drain them. Set the pot back on the stove over low heat, then put the potatoes back in the pot to steam out a little. This removes excess moisture. Shake the pan a little to prevent the potatoes from sticking too much. Add the butter and mash them. Mash well, adding cream or milk as needed to make them nice and smooth. Add salt to taste, turn off the heat, and set this aside.
In a large sauté pan, add the 1/4 cup of butter over medium heat. When it's hot, add the leeks and cook, stirring often, until they're soft, but not browned.
Add the flour to the pan with the leeks, mixing well. It will seize up, and that's OK. Keep it moving for a couple minutes to cook off the raw flour taste. Slowly pour in the wine or beer, stirring constantly. It will make a paste in the pan. Now add the milk-cream mixture you cooked the fish in little by little, stirring to combine. You want a sauce that is like thick gravy. You might not need all the mixture.
Add the poached fish, smoked fish and seafood, along with the mustard, horseradish and herbs. Stir and cook this for a couple minutes to warm through, then turn off the heat.
To make the pie or pies, spoon the filling into your ramekins, baking dishes or any shallow, oven-proof pan. Top with the mashed potatoes, then sprinkle the cheddar cheese on top. Since everything is cooked, you need only brown the topping in a toaster oven set to 400°F or put the pie or pies under the broiler. Be sure to put them on a baking sheet because the cheese often spills over the sides.
Notes
Keep in mind that you can use almost any combination of fresh and smoked fish or seafood, as well as whatever shellfish you feel like -- or you can leave out any of these elements.
Keys to Success
Sometimes I like to add another tablespoon of prepared horseradish to the mashed potatoes, or some roasted garlic, to kick them up a notch.
If you don't like cheddar, any other cheese that melts will do. Muenster or Swiss or, even better, gruyere, are all good options.
Alternate herbs could be rosemary, thyme, savory, lovage or chervil.