Seafood Gratin Shells

📝 Ingredients

• 400 g cod fillet
• 250 g raw peeled prawns
• 250 g button mushrooms
• 1 shallot
• 800 g potatoes
• 500 ml semi-skimmed milk (150 ml for the mash, 350 ml for the sauce)
• 25 g unsalted butter (10 g for the mushrooms, 15 g for the mash)
• 25 g cornflour (about 3 tbsp)
• 1 fish stock cube + 250 ml hot water
• 1 egg
• 60 g grated Emmental
• ½ lemon (juice)
• Salt, black pepper, nutmeg

🍴 Utensils

• 2 saucepans
• Frying pan
• Whisk
• Potato masher
• Slotted spoon
• 4 individual gratin dishes

📖 Steps

1. Peel the potatoes, cut them into evenly sized pieces and put them in a saucepan of cold salted water. Bring to a boil, then cook for 20 min over a medium heat, until the tip of a knife slides in without resistance.
2. Meanwhile, finely chop the shallot and slice the mushrooms. Fry them for 6 to 8 min over a medium heat in a frying pan with 10 g butter, until the released liquid has evaporated and they start to brown. Season with salt and pepper and set aside.
3. Dissolve the stock cube in 250 ml hot water and bring to a simmer. Cut the cod into 3 cm cubes, poach them for 2 min with the lid on, then add the prawns for 1 min, just until they turn pink: they'll finish cooking in the oven. Lift out with a slotted spoon, keeping the stock.
4. Mix the cornflour into 350 ml cold milk, stirring well to avoid lumps, then pour into the hot stock. Bring to a boil, whisking constantly, and cook for 2 min over a low heat, until the sauce coats the spoon. Off the heat, add the lemon juice and season with pepper and nutmeg.
5. Preheat the oven to 220 °C. Drain the potatoes and mash them with 15 g butter and 150 ml milk warmed for 1 min in the microwave. Mix in the beaten egg, a pinch of nutmeg and some salt: the mash should stay firm enough to hold as a border without slumping.
6. Gently mix the cod, prawns and mushrooms with the sauce, without breaking up the fish. Taste and adjust the seasoning: the sauce should be well seasoned, as the mash and cheese will mellow it.
7. Spoon or pipe a border of mash around the edge of 4 individual gratin dishes, then mark ridges with a fork. Divide the seafood filling between the centres and sprinkle with grated Emmental, especially over the sauce.
8. Bake for 12 to 15 min at 220 °C, until the cheese is browned and bubbling and the mash border golden. For more colour, put under the grill for 2 min, keeping an eye on it. Leave to rest for 3 min before serving: the sauce is scalding hot.