Cod and Pea Fish Pie

Heat the oven to 425°F with a rack in the middle. Wash your hands and clean the work surface. Scrub the potatoes under running water, then peel and cut them into 1-inch chunks. Rinse the onion, peel it, and finely dice it. Pat the fully thawed cod thoroughly dry with paper towels, cut it into 1 1/2-inch pieces, and refrigerate until needed. Wash your hands, knife, and cutting board after handling the raw fish.

Put the potatoes in the large saucepan with 8 cups water and 1/2 teaspoon salt. Bring to a boil, then reduce the heat and simmer for 15–20 minutes, until the potatoes are easily pierced with a fork.

Meanwhile, melt 3 tablespoons butter in the skillet over medium-low heat. Add the onion and cook for 8–10 minutes, stirring frequently, until soft but not browned. Stir in the flour and cook for 2 minutes.

Gradually whisk in 1 1/2 cups milk until smooth. Bring to a gentle simmer and cook for 3–5 minutes, stirring frequently, until thick enough to coat a spoon generously. Stir in the mustard, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Remove from the heat and stir in the frozen peas.

Drain the potatoes thoroughly. Return them to the saucepan over low heat for 1–2 minutes, shaking gently to evaporate excess moisture. Remove from the heat and mash with the remaining 2 tablespoons butter. Gradually mix in the remaining 1/2 cup milk, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Keep the mash thick and spreadable; avoid beating it, which can make it gluey.

Fold the raw cod pieces into the sauce and peas, then immediately transfer the filling to the baking dish. Spread it evenly. Spoon the mashed potatoes over the filling in small mounds, then gently join them together to cover the surface and reach the edges. Roughen the topping with a fork to create ridges that will brown.