Bring a medium-sized pan of water to the boil and add the potatoes. Boil for 10-12 minutes, until tender.
Meanwhile, bring another pan of water to the boil. Turn down to a simmer, then poach the salmon fillets for 5 minutes until cooked through. Transfer to a plate and leave to cool slightly, then flake the fish with a fork.
Mash the potato with a little freshly ground black pepper. Mix with the salmon then add half the parsley, the spring onions, 3 tsp capers, the mustard, lemon zest and flour.
Shape into 8 equally sized patties and coat in the breadcrumbs. Heat the oil in a large pan and cook the fishcakes for 2-3 minutes each side until crisp and golden.
To make the salad, mix together the remaining parsley, capers, lemon juice and crème fraîche. Stir in the shredded Brussels sprouts and tomatoes. Serve the fishcakes with the salad leaves, crème fraîche sauce and lemon wedges. Season with freshly ground black pepper.