Photograph: Jonathan Gregson
Preheat the oven to 220°C, fan 200°C, gas 7. Put the fennel, apricots, onion and watercress in a bowl; set aside.
Arrange the mackerel fillets in a lightly oiled shallow roasting tin; season and roast for 6-8 minutes.
Whisk all the ingredients for the dressing in a bowl; season to taste.
Add the mint leaves to the salad, pour over the dressing and toss together. Serve with the mackerel.
Get ahead: prepare the salad and dressing up to a few hours ahead. Store separately, cover and chill; bring back to room temperature before serving.