Preheat the oven to 200°C, fan 180°C, gas 6. Sit one of the salmon fillets skin-side down on a large sheet of baking parchment sat on a large sheet of foil. Finely chop the dill, parsley and capers together and sprinkle over the side of salmon. Slice the lemon thinly and arrange over the top of the herbs.
Sit the second salmon fillet on top, skin-side up. Tie the two sides of salmon together in four places with string to secure. Pop 1 bay leaf under each piece of string. Make up 500ml of stock using the stock cube and carefully pour 150ml over the salmon, then fold up the parchment and foil and scrunch the top to secure into a parcel. Save the rest of the stock for another recipe.
Place on a large baking tray and bake for 50 mins until the salmon is cooked through – check by poking a knife into the fish to make sure the salmon flakes easily.
Meanwhile, snip off most of the stalks from the watercress (reserving a few sprigs for garnish) and place the leaves in a food processor with the crème fraîche, Dijon mustard and the juice of 1⁄2 lemon. Process until smooth.
Carefully lift the cooked salmon onto a board. Reserve the cooking liquor in the bottom of the parcel.
Slice the salmon into approximately 150g portions and serve with the watercress sauce and a spoonful of the cooking liquor drizzled over. Also serve with cooked new potatoes, carrots and fine beans. Dress the plates with the extra sprigs of watercress.
With the leftover salmon, make our fishy fish cakes for your baby or toddler.