Salmon en croûte

Name: Salmon en croûte
Prep time: 30 min
Cook time: 40 min
Total time: 1 h 10 min
Cuisine: French, Mediterranean, European
Ingredients: 50g flaky sea salt; 25g demerara sugar; 2 x 500g skinless, boneless salmon fillets; 75g watercress chopped; 200g cream cheese; 2 tbsp fresh dill sprigs, chopped; 1 lemon zested and juiced; pinch smoked salt (optional); 500g block all-butter puff pastry; flour for dusting; 2 eggs yolks only, beaten; 1 cucumber peeled, halved, deseeded and sliced; 1 red onion finely sliced; 100ml cider vinegar; 1 tbsp sugar