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Heat the oven to 230C/210C fan/gas 8 and line a large, shallow roasting tin with baking parchment. Tip the potatoes into a large bowl and drizzle with the olive oil, then toss well to coat. Sprinkle over the parmesan, thyme and seasoning, then toss again to cover. Tip the potatoes into the tin and spread out in an even layer. Roast for 40 mins, turning after 30 mins, until golden and just cooked through.
Meanwhile, roughly chop the watercress and tip into a food processor or mini chopper along with the soft cheese, lemon zest and juice, the Dijon mustard or horseradish sauce and plenty of seasoning. Blitz until you have a thick, bright green sauce. Scrape into a bowl and set aside. Will keep covered in the fridge for up to a day.
Lay the salmon out on a board, skin-side down. With your hand positioned flat on top of the fillet, carefully use a long, sharp knife to slice into it from the side, being careful not to cut straight through. Open the fillet out like a book, then spread over half the sauce to cover the surface completely. Close the fillet again to enclose the sauce. Set aside until the potatoes are ready.
After 40 mins, toss the potatoes again and push them to the sides of the tin, leaving space in the middle for the salmon. Nestle the salmon into the middle of the potatoes, skin-side down. Drizzle with the olive oil and roast for 15-20 mins until the salmon is just cooked through. Serve straight from the tray with a few sprigs of fresh watercress scattered over and the remaining watercress sauce on the side for drizzling.