Cook the broccoli by steaming until just tender, about 4 minutes. Let cool and set aside. In an enamel or stainless steel bowl combine well the blue cheese, lemon zest, lemon juice, oil and salt and stir into the sour cream. In a salad bowl pour about half of the dressing over the florets, toss gently, and chill for 2-3 hours. Just before serving, arrange the florets and tomatoes on salad greens on a chilled platter and serve with the remaining dressing.