Fish pies -casseroles, in truth -are a week-night staple in Ireland and England. The original recipe calls for smoked haddock- you can use a good-quality smoked white fish instead, making sure to get rid of the small bones.
For the filling/topping
680g potatoes, peeled and cut into same-size chunks
900g smoked fish, such as haddock or whitefish, skinned and boned (see headnote)
1 bay leaf
1 lemon, cut into wedges
1 small onion, cut in half (peeled)
1 to 2 tbsp unsalted butter
1 to 2 tsps full cream milk or heavy cream
Freshly ground black pepper
1 large egg, beaten, for the potato crust
For the sauce
16 tablespoons (2 sticks) unsalted butter
110g (scant 1 cup) flour
1 cup whole or low-fat milk
1/2 cup heavy cream
2 small onion, finely chopped
Freshly ground black pepper
3 tomatoes, cored and chopped
3 large hard-cooked eggs, coarsely chopped
1 teaspoon chopped fresh parsley, plus more for garnish
Preheat the oven to 190 degrees.
Use cooking oil spray to grease a large, deep pie plate or a 33 x 23cm baking dish. (Depending on the size of your pie plate, you might also need a few ramekins for baking any excess mixture.)
For the filling/topping: Bring a large pot of salted water to a boil over high heat. Add the potatoes and reduce the heat to medium; cook for 10 to 15 minutes, until tender.
Meanwhile, place the smoked fish in a large saucepan along with the bay leaf, a few of the lemon wedges and the onion halves.
Cover with cold water; bring to a boil over medium-high heat and cook for 8 to 10 minutes.
Strain through a fine-mesh strainer; transfer the fish to a plate and reserve at least 2 cups of the cooking liquid in the saucepan. Discard the remaining solids in the strainer.
Drain the potatoes, then mash them with the butter (to taste) and the milk or cream.
Season lightly with salt and pepper; this mixture will be the topping for the pie.
For the sauce: Melt the butter in a separate large saucepan over low heat. Whisk in the flour and cook, stirring, for 2 minutes to form a roux.
Add the milk and cream to 1 1/4 cups of the reserved cooking liquid (from the smoked fish), then pour that mixture gradually into the roux, whisking to create a thin sauce. Increase the heat to medium-high; cook just until the mixture begins to boil, stirring constantly, then reduce the heat to medium-low and add the onions.
Cook, stirring, for a minute or two; the sauce will thicken. (If it thickens too much, add some of the remaining cooking liquid.) Taste, and season with salt and pepper as needed.
Stir in the tomatoes, hard-cooked eggs, teaspoon of parsley and the drained smoked fish, then pour evenly into the baking dish.
Pipe or dollop a border of the potato mixture at the perimeter of the pie, then brush the piped border with the beaten egg.
Bake for 30 minutes, until just golden brown.
Sprinkle with parsley.
Serve hot, with the remaining lemon wedges.