Preheat the oven to 200°c, 400°F, gas mark 6.

Boil the potatoes in a large saucepan of salted water for 10-15 minutes until tender.

Place the chicken on a roasting tray in a pre-heated oven and heat through for 15 minutes.

Meanwhile, blend the watercress with the lemon and wholegrain mustard in a food processor. Add the olive oil and season.

Toss the cooked potatoes and the salad leaves together with the dressing and place in a large serving bowl.

Slice the avocado and arrange with the chicken on top of the salad. Serve with the watercress and lemon dressing drizzled over.