Put the salmon in a microwave-proof dish, cover with cling film, then cook on High for 3½-4 mins until the salmon is just cooked. Peel the skin away and flake into large chunks.
Toss together the watercress, chickpeas, fennel and onion, then arrange on a large platter. Zest one of the oranges, cut away the skin and pith from both fruit, then segment them both. Squeeze the juice from the middles of the oranges into a bowl and mix 2 tbsp of it with the yogurt, dill, zest and seasoning.
Scatter salmon over the salad, then serve drizzled with the creamy orange dressing.