Remove the outer green leaves and break the cauliflower neatly into sprigs. Trim away any long pieces of stalk and add the sprigs to a pan of boiling salted water. Simmer for 8–10 minutes until the sprigs are tender but not broken. Test by piercing the stalks with the point of a sharp knife. Drain and place in a mixing basin. Pour over half the dressing while the cauliflower is still warm – this prevents the cooked cauliflower going dark in colour.
Meanwhile, hard-boil the eggs, drain and plunge into cold water. When cold shell and cut into quarters. Add the egg to the cauliflower and toss with a little extra dressing if necessary. Wash the watercress well and remove the long stalks. Arrange round the outer edge of a salad bowl. Spoon the cauliflower and egg into the centre. Pour the remaining dressing over the watercress and serve.