Whisk together vinegar, oil, mustard, salt and pepper until fully combined.
Place the pears, trout, fennel, watercress and chives in a large serving bowl.
Pour over the dressing, toss lightly and serve.
This recipe is best made just prior toserving. If this is inconvenient, drizzle a little lemon juice over the sliced pears and fennel to stop browning and maintain colour.