Heat the oven to 180C. Put the potatoes on to cook in a pan of lightly salted boiling water. When they have been simmering for about 10 minutes, start to cook the chicken.
Measure 2 teaspoons of the olive oil into a large frying pan or wok. Heat it, then add the chicken pieces but don’t stir them immediately – just let them sear for a few seconds over a high heat so they get nice and brown. Now turn them over and sear on the other side.
Add the tomatoes, olives and rosemary sprigs to the pan with the chicken. Remove from heat and put into the oven for 30 mins. Season the mixture with black pepper.
Meanwhile, make the dressing by mixing the rest of the olive oil together with the lemon zest, lemon juice and mustard. Season with a little salt and pepper.
Strew the watercress over 4 serving plates. Drain the potatoes then spoon them over the watercress. Crush each potato lightly with a fork. Share out the chicken mixture, piling it on top of the potatoes, then drizzle the lot with the olive oil and lemon dressing.