Step 1 Preheat oven to 200°C. Line a large baking tray with baking paper. Place sweet potato on tray and spray with olive oil. Roast for 25–30 minutes, or until golden and tender, then set aside.
Step 2 Coat salmon fillets with an even sprinkling of sumac. Spray a large non-stick frying pan with olive oil; set over high heat. Cook salmon for 3 minutes per side, or until cooked to your liking. Set aside to cool; break into large flakes.
Step 3 Bring a medium saucepan of water to the boil. Immerse green beans to blanch until just tender. Refresh beans under cold running water; drain well. Place reserved sweet potato, beans, watercress and tomato in a large bowl; toss.
Step 4 Mix yoghurt, chopped chives and lemon juice in a small bowl.
Step 5 Divide salad among 4 plates, top with salmon and drizzle with yoghurt dressing. Serve salad with a lemon wedge.