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The day before you want to assemble the en croûte, mix the salt and sugar in a bowl. Scatter half of the mix over a tray, then lay one of the salmon fillets on top, skinned-side down, and scatter with more of the salt mix. Lay the second fillet on top, skinned-side up, and sprinkle over the rest of the salt mix. Cover, then place another tray on top. Weigh the tray down with a couple of tins and put in the fridge for up to 48 hrs, or for at least 12 hrs (the longer you leave it, the firmer the salmon will be).
When you’re ready to assemble the en croûte, unwrap the salmon fillets, rinse them in cold water and dry with kitchen paper, then set aside. Put the watercress, cream cheese, dill, lemon zest and juice, smoked salt (if using) and a generous grinding of pepper into a bowl and mix well, then set aside.
Line a baking tray with parchment. Roll out half of the pastry on a lightly floured surface so it’s 2.5cm larger in diameter than the salmon fillet, then drape it over the lined baking tray. Place one of the fillets on the pastry, skinned-side down, and spread with the cream cheese mix, then lay the second fillet on top, skinned-side up. Brush the edge of the pastry with a little of the beaten yolk. Roll out the rest of the pastry to fit, then drape it over the salmon and tuck it in at the sides. Trim the edges and crimp or press with a fork to seal. Brush with more beaten yolk, then put the en croûte in the fridge for 30 mins before using a spoon to create a scale-like effect along the top of the pastry. Chill for up to 24 hrs or for at least another 30 mins.
Heat oven to 220C/200C fan/gas 7. Bake for 20 mins, then glaze with the remaining yolk. Lower the heat to 180C/160C fan/gas 4 and bake for 20 mins more, then remove and leave to stand for 10 mins.
To make the pickle, put the cucumber and onion in a bowl. Tip the vinegar, sugar and a pinch of salt into a pan and bring to the boil. Once the sugar has dissolved, pour over the veg, toss well and set aside. Can be made up to two days ahead and kept in the fridge. Slice the en croûte into six portions and serve with the pickled veg on the side.