Salmon en Croûte with Spinach and Gruyère

Pat the salmon dry with kitchen paper. Season all over with ½ tsp salt and the black pepper. Spread the Dijon mustard evenly across the top surface and set aside. In a bowl, combine the crème fraîche, Gruyère, tarragon, parsley, and lemon zest; stir well and season with the remaining ½ tsp salt and a squeeze of lemon juice.

Melt the butter in a wide pan over medium heat. Add the shallots and cook, stirring occasionally, until soft and translucent, about 4 minutes. Add the garlic and cook until fragrant, about 1 minute.

Add the spinach to the pan in two or three batches, turning with tongs until fully wilted, about 2 minutes total. Tip into a colander and press firmly with the back of a spoon to squeeze out as much liquid as possible. Spread on a plate and allow to cool completely.

Dust the work surface lightly with flour and roll the puff pastry into a rectangle approximately 35 cm × 45 cm. Cut it into two pieces: one slightly larger than the salmon fillet (the base) and one about 4 cm wider and longer on all sides (the lid). Transfer the base piece to a baking-paper-lined tray.

Spread the cooled spinach mixture over the pastry base, leaving a 3 cm border. Spoon the crème fraîche mixture evenly over the spinach. Lay the salmon fillet, mustard-side down, on top. Brush the pastry border with beaten egg. Drape the pastry lid over the salmon and press the edges firmly together, then crimp with a fork to seal. Brush the entire surface with beaten egg, score a light cross-hatch pattern with a knife without cutting through, and cut two small steam vents in the top.

Cover loosely with cling film and refrigerate for at least 30 minutes, or up to 4 hours — this chilling step is essential for keeping the pastry crisp and laminated rather than soggy once baked.

Preheat the oven to 210°C, or 190°C for a fan oven, or gas mark 7.

Bake on the middle shelf until the pastry is deep golden brown and a meat thermometer inserted through a steam vent into the thickest part of the salmon reads 63°C, approximately 28–32 minutes.

Remove from the oven and rest uncovered for 5 minutes before slicing and serving.