Employ a mix of fishes, if you like. A few ounces of smoked fish would be sublime. Variety is wise in a pie like this. Note, too, the roux measurements make a lot of roux. It is easier to cook a lot of roux than it is to cook a little. After, you’ll have leftover roux in the fridge. Always handy.
- Make the roux: Melt the butter in a saucepan over medium heat, then add the flour and cook, stirring occasionally, until the flour’s raw scent slips away, about 2 minutes.
- Make the mashed potatoes: Put the potatoes into a saucepan along with a hefty pinch of salt, cover with cold water, and bring to a boil. Once at a boil, reduce the heat to a sturdy simmer and cook until the potatoes are about half-cooked, about 15 minutes. Strain off two-thirds of the remaining liquid, cover the saucepan, reduce the heat to low, and steam the potatoes until they are fully cooked.
- In a small saucepan, bring the milk to a simmer. Meanwhile, peel the potatoes by pulling off their skins. Mash the flesh using a potato masher or the paddle attachment of a standing mixer. Beat the egg yolks into the mashed potatoes and add the simmering milk in stages. You want just enough to give the mashed potatoes a soft, light consistency. Beat in the butter, adding the amount based on how rich you want the result to be. Taste and season with salt and pepper.
- Make the fish pie: Cut the fish into pieces that are about 5 ounces each. Season with salt and pepper.
- Add ½ tablespoon butter to a skillet and heat over low heat. Add the onion and cook, stirring occasionally, until the the onion is soft and translucent but not browned. Transfer the onion to a plate. Increase the heat, add a bit more butter, then the mushrooms in batches, cooking each batch until the mushrooms are shrunken and lightly browned. Don’t crowd the skillet. Season the cooked mushrooms with salt and pepper and add them to the onion.
- In a skillet large and wide enough to hold both the fish and the 2 cups milk, add the fish pieces in a single layer, more or less, then cover with the milk and add the bay leaf. Season to taste with salt and pepper. Cover and simmer gently until the fish is barely cooked, about 4 minutes. Using a slotted spoon, transfer the fish to a plate. Carefully remove all bones and skin. Discard the bay leaf.
- Preheat the oven to 350℉.
- Bring the fish-cooking liquid to a boil and thicken it by whisking in a little roux at a time. Start with a tablespoon. How thick you want it is up to you. A good barometer for thickness is melty cheese dip. Add the heavy cream, if using, and the thyme, parsley, mushrooms, onion, fish pieces, and peas. Stir gently, taste, and adjust the seasoning. Spoon into one large baking dish or 6 to 8 small ones. Pipe or scoop the mashed potatoes on top. The pie can be prepared ahead up to this step and refrigerated for about a day.
- To finish, bake the pie—or pies—in the preheated oven for 10 to 15 minutes if the filling and topping are warm, 30 minutes if reheating the dish. Use the broiler to brown the top, if necessary.
Scott Hocker is a writer, editor, recipe developer, cookbook author, and content and editorial consultant. He has worked in magazines, kitchens, newsletters, restaurants and a bunch of other environments he can’t remember right now. He has also been the editor in chief of both liquor.com and Tasting Table.