Preheat the oven to 170°C, fan 150°C, gas 3. In a bowl, mix together the pistachios, breadcrumbs, 1⁄2 of the parsley and the lemon zest.
Place the salmon, skin-side down, diagonally on a large sheet of baking paper on a baking tray. Spread the soft cheese over the salmon, leaving a 2cm border all around. Press a layer of the pistachio mixture on top of the soft cheese. Fold up the baking paper and secure with kitchen string to make a sealed parcel. Bake in the oven for 30-35 minutes, or until just cooked through and starting to flake.
Meanwhile, put the potatoes in a large pan of boiling water and cook for 20 minutes, until tender. Drain, tip into a serving bowl and scatter over the remaining parsley.
Whisk together the capers, lemon juice and sugar in a small bowl, then slowly whisk in the oil.
Transfer the salmon to a platter and arrange the beetroot slices, cucumber and watercress around it. Drizzle the dressing over the salad and serve with the potatoes.