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Ingredients
Method
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Begin desalting the cod the evening before. Place the pieces in a large bowl, cover completely with cold water, and refrigerate. Change the water after 8 hours and again before you start cooking. After 24 hours, taste a small flake of the raw fish; it should be pleasantly saline but not sharp. If it is still very salty, soak for another 2 to 3 hours.
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Preheat the oven to 180°C fan (200°C conventional). Grease a deep ovenproof dish approximately 30cm x 20cm with butter.
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Drain the cod and pat dry. Place in a wide pan, cover with the milk, and bring to a very gentle simmer over low heat. Poach for 8 minutes until the flesh is just opaque and starts to flake at the thickest point. Lift the fish out carefully and set aside on a plate. Reserve the poaching milk.
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Once the cod is cool enough to handle, remove any remaining bones and skin. Break the flesh into large flakes, roughly 3 to 4cm. Do not shred it finely; you want pieces that hold their shape in the finished dish.
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Cook the potato slices in a pan of unsalted boiling water for 6 minutes. They should be just starting to soften but still holding their shape. Drain and set aside. Do not salt the water; the cod will season the whole dish.
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Melt the butter with the rapeseed oil in a heavy frying pan over medium heat. Add the leeks and garlic and cook for 7 to 8 minutes, stirring occasionally, until the leeks are soft and translucent but not browned. Stir in the flour and cook for 1 minute, scraping the base of the pan.
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Pour the reserved poaching milk into the leek mixture gradually, stirring constantly to avoid lumps. Add the double cream. Bring to a gentle simmer and cook for 3 minutes until the sauce has thickened enough to coat the back of a spoon. Stir in both mustards and a generous amount of black pepper. Taste the sauce at this point; if it needs salt, add it carefully given the cod.
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Arrange half the potato slices across the base of the greased dish, overlapping slightly. Scatter half the flaked cod over the potatoes. Pour over half the leek and cream sauce. Repeat with the remaining potatoes, cod, and sauce.
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Scatter the grated cheese evenly over the top. Place the dish on the middle shelf of the oven and bake for 25 minutes. The top should be golden and blistered in places, and the sauce should be bubbling at the edges. If the cheese colours too quickly before the 25 minutes are up, lay a sheet of foil loosely over the top.
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Remove from the oven and rest for 5 minutes before serving. Scatter the chopped parsley over the top just before bringing the dish to the table. Serve directly from the dish.
Irish Context
Trosc is the Irish word for cod, and dried salted cod was a practical staple in coastal Irish households long before refrigeration. The fish was preserved at sea and could be stored for months.
This recipe takes that same salt cod and uses it in a baked gratin format that makes use of the poaching liquid rather than discarding it, so none of the flavour drawn out during soaking goes to waste. The pairing of leek, potato, and mustard cream is not a reconstruction of any single historical recipe; it is an honest combination of ingredients that are genuinely available and used in Irish kitchens today.
Tips
The quality of the desalting process determines the whole dish. If you skip changes of water or rush the timing, the finished bake will be too salty and no amount of cream will fix it.
Set a reminder on your phone for the water changes. Floury potatoes such as Rooster or British Queens work far better here than waxy varieties.
Waxy potatoes stay firm and separate from the sauce; floury ones start to absorb the cream at the edges and bind the layers together as the dish bakes. If the sauce looks thin when you assemble the dish, do not worry.
The starch from the parboiled potatoes thickens it considerably during the oven time. Salt cod varies in thickness depending on which part of the fish it comes from.
Thin tail pieces will cook faster during poaching; check them at 5 minutes. Thicker loin pieces may need 10 minutes.
Leftovers reheat well covered in foil at 160°C fan for 15 minutes with a splash of milk added to the corner of the dish to prevent the sauce from splitting.
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