Preheat the oven to 200°C, fan 180°C, gas 6. Cut the peppers into pieces.
Brush half the oil onto the skin of the salmon and put the fish on a baking tray. Sprinkle the beans and peppers around the salmon and roast for 10 mins or until the salmon is cooked through.
Scatter the watercress onto plates, top with the beans and peppers, then flake over the salmon. Drizzle over the remaining olive oil and the lemon juice. Season with freshly ground black pepper and serve.