Fish Pie with Cod and Salmon under Mashed Potatoes

If using frozen fish, thaw it completely in the refrigerator beforehand. Check the fillets and remove any remaining bones, then pat the fish thoroughly dry with paper towels. Cut both fillets into pieces as even as possible, about 3–4 cm, and keep refrigerated until assembly. Cut the peeled potatoes into pieces about 3 cm and finely dice the onion. Have ready an ovenproof dish with a capacity of about 3.5 l, roughly 30 × 22 cm and 6–7 cm deep; it does not need greasing.

Place the potatoes in a pot, add just enough cold water to cover them, and bring to a boil. Cook gently for about 20–25 minutes from when the water comes to a boil, until a fork easily enters the center of a piece. Drain off all the water thoroughly, return the pot to low heat, and steam-dry the potatoes for about 2 minutes, shaking the pot. Do not brown them. Make the sauce in a separate saucepan while the potatoes cook.

Preheat the oven to 200°C with conventional top and bottom heat. In a separate saucepan, melt the 40 g butter allocated to the sauce. Add the onion and cook over medium-low heat for about 6–8 minutes, stirring, until softened but not browned. Add all 40 g flour and stir for about 2 minutes to combine it with the butter and onion.

Gradually pour in the 500 ml milk allocated to the sauce, whisking constantly. Bring to a gentle boil and cook over low heat for about 3–4 minutes, stirring along the bottom as well. Add 5 g salt, all the pepper, and the nutmeg. The sauce should be smooth apart from the onion pieces, distinctly thick, and coat a spoon in a thick layer. Keep it over very low heat, stirring occasionally, while finishing the mashed potatoes; do not add the fish yet.

Mash the steam-dried potatoes with a potato masher or pass them through a potato ricer. Add the remaining 40 g butter, remaining 100 ml milk, and remaining 5 g salt. Mix briefly until the butter melts and the mashed potatoes are smooth, thick, and spoonable. Do not use a blender and do not add any extra liquid. Start assembling immediately so the potatoes stay warm.

Remove the hot sauce from the heat, add the raw cod and salmon chunks, and gently coat them in the sauce. Immediately transfer everything to the ovenproof dish, distributing the fish evenly in as close to a single layer as possible. Spoon small portions of warm mashed potatoes across the entire surface, then gently join them together with a spoon, without pressing them into the fish. Cover the filling right to the edges and make ridges on top with a fork. Leave about 1–2 cm of space below the rim for bubbling. Put the dish straight into the preheated oven; do not set the assembled pie aside to bake later.

Bake without a cover on the middle rack for about 35–40 minutes. The sauce should be bubbling clearly around the edges, and the potato ridges should be browned. After this time, part the mashed potatoes in the center and check one of the thicker pieces of fish: the center of the dish must be hot, and the fish cooked all the way through, separating easily into flakes, without a raw, glassy center. If the middle needs more baking, bake for another 5–10 minutes; loosely cover the top with foil if it is already deeply browned. Do not stop baking just because the potatoes have browned.

Take the dish out of the oven and let it rest for about 10 minutes so the sauce thickens slightly and the servings are easier to spoon out. Divide into four portions and serve with a deep spoon, taking fish, sauce, and the potato layer in each serving. Serve the pie warm without any extra garnish.