Fish Pie with Sweet Potato Mash

A staple in Irish and Irish-American households, fish pie is usually topped with a purée of white potatoes. But swap white potatoes for sweet potatoes, and it turns out that the mild brininess of the fish is fantastic with the slightly sweet topping. Feel free to substitute salmon or a mild white-fleshed fish for the cod, and shucked clams for the scallops. I find that piping the sweet potato purée onto the casserole is the easiest way to seal in the filling, though carefully spreading it with some patience also works.

Sweet Potato Purée:
1½ lbs orange-fleshed sweet potatoes
1 knob butter

Filling:
½ lb fresh cod or haddock
½ lb large shrimp
½ lb bay scallops
6 Tbsp. (¾ stick) butter, at room temperature
1 small onion, finely chopped
2 cups whole milk
2 bay leaves
2 sprigs fresh thyme
sea salt
black pepper, freshly grated
2 Tbsp. all-purpose flour, unbleached
¼ cup fresh chives, chopped

Topping:
2 cups sweet potato purée

Sweet Potato Purée:
Peel the sweet potatoes and cut them into 1" cubes. Put the sweet potatoes in a large saucepan and add cool water to cover by 1". Set the pan over medium-high heat and bring to a boil. Reduce the heat to low to maintain a gentle simmer and cook until the sweet potatoes are fully tender, about 15 minutes. Drain the sweet potatoes, reserving the cooking liquid.

For a perfectly smooth purée, first pass the sweet potatoes through a food mill or press them through a potato ricer, discarding any pulpy, stringy flesh that remains. Give the milled sweet potatoes a good stir with a spoon, and add a Tbsp. or two of cooking liquid if the purée seems dry or tacky. For a coarser purée, simply mash the sweet potatoes with the back of a fork or a potato masher. They will collapse into a relatively smooth mass that should not require additional liquid to remain moist. To store, let the purée cool and then refrigerate it in an airtight container for up to 3 days. To freeze, place cupfuls of purée in sandwich-size plastic zip-top bags, flatten the bags as you seal them, and stack them in the freezer.

Preheat the oven to 375°F (190°C/170°C fan, Gas Mark 5).

Cut the cod into 1" pieces. Peel and devein the shrimp. Pick over the scallops for bits of shell, and if there is a tough band of muscle adhered to one side of each scallop, remove it. Refrigerate the seafood until ready to use.

Melt 2 Tbsp. of the butter in a medium saucepan set over medium heat. Add the onion and cook, stirring, until translucent and soft, about 5 minutes. Pour the milk into the pan and add the bay leaves and thyme sprigs. Bring to a gentle simmer, stirring often. Stir ½ tsp. of salt into the milk.

Poach each type of seafood individually: Place the cod in the simmering milk and cook until the fish is nearly cooked through, 1-2 minutes. Lift the cod from the milk with a slotted spoon and place it in a 2-3 qt casserole dish. Add the scallops to the milk and poach until opaque, about 30 seconds. Lift from the milk with a slotted spoon and add to the dish with the fish. Add the shrimp and poach until they just begin to curl, about 1 minute. Lift them from the milk with a slotted spoon and add them to the dish. Use your hands or a fork to flake the cod into bite-size pieces. Season the fish with ½ tsp. salt and ¼ tsp. pepper.

Béchamel Sauce:
In a small bowl, mash 2 Tbsp. of the butter with the flour. Remove the bay leaves and thyme sprigs from the milk. Bring the milk to a simmer once again and whisk the butter mixture into the milk until no lumps remain. Reduce the heat to low and simmer, stirring, until the mixture thickens, about 5 minutes.

Filling:
Pour the béchamel over the fish in the casserole dish. Add the chives and fold gently to combine everything. Taste the filling and adjust the seasoning with salt or black pepper, if needed.

Topping:
Reheat the sweet potato purée in a pot, stirring constantly, until warm. Stir the remaining 2 Tbsp. butter into the sweet potatoes until it melts and season lightly with salt. If desired, transfer the warm sweet potato purée to a piping bag fitted with a star tip. Pipe or spread the purée over the filling, sealing the edges.

Bake until the filling is bubbly and the topping is browned, about 20 minutes.

Let stand for a few minutes before serving.