Cook the pasta in a large pot of lightly salted boiling water for 10 minutes. Add the fava beans and bring back to a boil, then reduce the heat slightly and cook for a further 2-3 minutes until the pasta is tender and the beans are lightly cooked.
Meanwhile, mix together the tomatoes, ham, chives and oil in a large serving bowl. Set aside. To make the pesto, process the watercress, walnuts and garlic together in a food processor or blender until finely ground. Add half the oil and pulse until combined, then add the remaining oil, lemon zest and juice and the yogourt and blend again. Season to taste.
Drain the pasta and beans and immediately add them to the tomato mixture. Mix well, then cover and leave to stand for 5 minutes. Stir the pesto and chopped watercress into the salad, then serve immediately.
Cook's tip: If prepared ahead, add the pesto and watercress just before serving. Until then, keep the rest of the salad and the pesto, covered separately, in the refrigerator.
Variations: For a vegetarian salad, omit the ham and add 4 chopped, hard-boiled eggs to the salad with the watercress. Instead of fava beans, use 250 grams (1⁄2 lb) small broccoli florets. Cook with the pasta in the same way.
Each serving: 24.5 g protein, 29 g fat of which 4 g saturates, 53 g carbohydrate, 11 g fibre, 596 Calories