Cut the sweet potatoes in half, then into thick wedges and place in a large roasting tin with the chicken, onion and tomatoes. Tuck the chorizo in and around the potatoes, then sprinkle with the rosemary and some salt and pepper. Drizzle with the oil.
Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for about 35 minutes, turning once or twice, until the chicken is golden and cooked through and the potato wedges are browned and tender.
Spoon on to warmed serving plates and serve with a watercress salad, if liked.