Salmon Orloff with Spinach and Mushrooms

Pat the salmon fillet dry with kitchen paper. Using a sharp knife, slice the salmon crosswise into 8 even tranches, cutting down to but not fully through the skin so the fillet stays hinged and intact. Season lightly with salt and pepper between the cuts and across the surface. Mince the shallots, garlic, and mushrooms. Pick the thyme leaves. Grate all cheeses.

Make the duxelles: melt 30 g butter with the olive oil in a wide skillet over medium-high heat until foaming. Add the shallots and cook, stirring frequently, until soft and translucent — about 3 minutes.

Add the garlic and thyme to the pan and cook, stirring, until fragrant — about 1 minute.

Add the mushrooms to the pan, season well, and cook over high heat, stirring frequently, until all moisture has evaporated and the mixture is dry and beginning to turn golden — about 10–12 minutes. Add the white wine and cook until completely absorbed — about 2 minutes. Season with ¼ tsp nutmeg, taste, and adjust salt. Transfer to a bowl and allow to cool completely.

In the same pan, melt the remaining 30 g butter over medium heat. Add the spinach in batches and cook, turning with tongs, until just wilted — about 2 minutes. Season with the remaining ¼ tsp nutmeg, salt, and pepper. Drain in a sieve, pressing firmly to remove all liquid, then roughly chop. Combine the spinach with the cooled duxelles and mix well.

Make the Mornay sauce: melt 40 g butter in a heavy saucepan over medium heat. Add the flour and cook, stirring constantly, for 2 minutes until the roux smells biscuity but has not coloured.

Add the warm milk to the roux in a slow, steady stream, whisking constantly, and cook over medium heat until the sauce thickens and coats the back of a spoon — about 5 minutes. Remove from the heat.

Stir the Gruyère, Parmesan, Dijon mustard, and egg yolk into the hot sauce until the cheese is fully melted and the sauce is smooth. Season with salt, white pepper, and nutmeg. Press a sheet of cling film directly onto the surface and set aside.

Preheat the oven to 190°C, or 170°C for a fan oven, or gas mark 5. Lightly butter a baking dish just large enough to hold the salmon snugly.

Open each cut in the salmon and spoon an even layer of the mushroom and spinach mixture between every two tranches, pressing gently to adhere. Reshape the fillet to its original form and transfer skin-side down to the prepared baking dish.

Pour and spread the Mornay sauce generously over and around the salmon, covering it completely. Scatter the finishing Gruyère and Parmesan evenly over the top and dot with the small cubes of butter.

Bake at 190°C until the top is deep golden and bubbling and the salmon reads an internal temperature of 63°C on a meat thermometer inserted into the thickest tranche — about 20–22 minutes. If the top needs more colour, place under a hot grill for 2–3 minutes.

Rest the dish for 5 minutes before serving. Spoon the sauce from around the fish over each portion and serve with steamed new potatoes and a crisp green salad.