Salmon and Spinach Pâté en Croûte

Mince the shallot, pick and finely chop the tarragon and dill, and zest the lemon. Remove every pin bone from both salmon fillets with tweezers, then cut the 400g piece into rough chunks. Place the salmon chunks, egg white, crème fraîche and the food processor bowl and blade in the freezer for 20 minutes — everything needs to be close to 0°C so the mousseline emulsifies without splitting. Sift the flour into a large bowl.

Make the hot-water crust pastry: melt the butter with the milk and water in a small saucepan over medium heat until the mixture reaches a rolling boil, then pour immediately over the sifted flour and salt, beating hard with a wooden spoon until a shaggy dough forms. Knead briefly until smooth, wrap, and set aside at room temperature for 30 minutes to relax.

Wilt the spinach: melt the butter in a wide frying pan over medium-high heat, add the shallot and cook for 2 minutes until softened, then add the spinach and toss for 1–2 minutes until fully wilted. Season with the nutmeg, salt and white pepper, then tip into a colander and press out as much liquid as possible with the back of a spoon — spread on a clean tea towel and press again, since wet spinach will steam the pastry and collapse the structure. Leave to cool completely.

Make the mousseline: blitz the well-chilled salmon chunks in the cold processor bowl for 30 seconds to a smooth paste, then, with the motor running, add the egg white through the feed tube and process for 20 seconds until fully incorporated. Add the crème fraîche, mustard, lemon zest, tarragon, dill, salt and white pepper and pulse just 5–8 times to combine — over-processing will split the emulsion. Scrape into a bowl set over ice and refrigerate until needed.

Preheat the oven to 200°C, or 180°C for a fan oven, or 400°F, or gas mark 6 and lightly grease a 20cm x 9cm terrine mould or loaf tin. Divide the rested dough into two-thirds and one-third. On a lightly floured surface, roll the larger piece to a rectangle about 35cm x 25cm and 4mm thick, then drape it into the mould, pressing well into the corners and leaving the excess pastry overhanging the sides.

Spread half the mousseline evenly over the pastry base. Lay the whole 280g salmon fillet lengthways down the centre, checking it is an even thickness so it cooks through evenly, then pack the wilted spinach firmly over and around it. Spoon over the remaining mousseline and smooth the top level with an offset spatula.

Roll the remaining pastry to a rectangle large enough to cover the top, lay it over the filling, and press the edges together with the overhanging base pastry, crimping tightly to seal. Cut a 1.5cm round chimney hole in the centre of the lid and insert a small roll of foil to keep it open during baking, then brush the whole surface with the beaten egg yolk and score a light diamond pattern across the lid with the tip of a knife, taking care not to cut through.

Bake on the middle shelf for 20 minutes until the pastry is set and just beginning to colour.

Reduce the oven to 170°C, or 150°C for a fan oven, or 325°F, or gas mark 3 and continue baking for 55–60 minutes. Insert a digital probe meat thermometer through the chimney hole into the very centre of the forcemeat: it must read 72°C — the target internal temperature for this salmon and spinach forcemeat, verified with the probe rather than guessed at, since an emulsified mousseline is treated like any ground or blended preparation rather than a whole fillet cooked to doneness. Remove from the oven once confirmed.

Transfer the pâté en croûte, still in its mould, to a wire rack to cool completely at room temperature — allow at least 2 hours — then refrigerate uncovered for a minimum of 4 hours, ideally overnight, before unmoulding or slicing; this rest is what lets the mousseline set firmly enough to cut clean portions.

Make the aspic (optional but traditional): bloom the gelatine in 2 tbsp of the fish stock for 5 minutes, then warm the remaining stock in a small saucepan over low heat, whisk in the bloomed gelatine and stir for 2 minutes until fully dissolved. Cool to room temperature until just beginning to thicken — it should pour but not yet set — then use a small funnel to pour it slowly into the chimney hole of the chilled terrine in stages, pausing between additions to let it settle until the cavity is full. Return to the fridge for 1 hour until the aspic sets.

To serve, unmould the pâté en croûte and slice into portions about 3cm thick with a sharp serrated knife. Serve with cornichons, Dijon mustard and dressed watercress.