Fish Pie with Blue Ling and Gratinated Mashed Potatoes (English Tradition)

Pasos

1. Preheat the oven to 200 °C (180 °C fan). Generously butter a baking dish measuring about 25×30 cm.
2. Prepare the fish: rinse the blue ling fillets under cold water and pat them dry with paper towels. Cut them into 4 to 5 cm pieces. Season lightly with salt and pepper. Set aside in the refrigerator.
3. Prepare the béchamel: in a medium saucepan, melt the butter over medium heat. When it foams, add the flour all at once and whisk vigorously with a wooden spoon for 1 to 2 minutes, until the roux turns pale blond and smells nutty. Pour in the cold milk in a thin stream, whisking constantly to prevent lumps. Bring to a boil while continuing to whisk, then lower the heat and simmer for 3 to 4 minutes until the sauce thickens and coats the back of a spoon. Season with salt, pepper, and a pinch of nutmeg. Remove from the heat and cover with plastic wrap pressed directly onto the surface to prevent a skin from forming.
4. Prepare the mashed potatoes: peel the potatoes and cut them into 3 cm cubes. Place them in a large saucepan and cover with cold salted water (about 1 teaspoon of salt per liter). Bring to a boil, then lower the heat and cook at a gentle boil for 15 to 20 minutes, until the cubes are tender (the tip of a knife should slide in without resistance). Drain thoroughly in a colander, then return the potatoes to the hot saucepan for 1 minute to dry them. Pass them through a potato ricer or a fine-mesh food mill. Add the butter cut into small cubes and the hot milk (reheat the milk in the microwave or in a small saucepan). Stir with a spatula until smooth and creamy. Adjust the salt and pepper. Cover and keep warm.
5. Assemble the fish pie: spread the fish pieces in a single layer over the bottom of the buttered dish. Thinly slice the onion and distribute it over the fish. Pour the béchamel over the fish and onion, making sure all the pieces are well covered. Spread the mashed potatoes on top, starting at the edges and smoothing the surface with an offset spatula. Sprinkle evenly with the grated cheddar.
6. Place the dish on the middle rack. Bake for 25 to 30 minutes, until the top is well browned and the sauce is visibly bubbling around the edges. If the top browns too quickly, cover it with aluminum foil during cooking. Let it rest for 5 minutes out of the oven before serving.
7. Garnish with chopped fresh parsley (a few whole leaves for presentation) and serve hot, straight from the dish.