Put the broccoli in a large steaming basket and steam over 1 inch of boiling water for 3 to 5 minutes, until tender but still crunchy. Drain and rinse immediately with cold water to stop the cooking. Drain, and transfer to a bowl and toss with the onion and tomatoes.In a small bowl, combine the orange jioce, vinegar, sugar and orange zest. Slowly whisk in the oil utnil well blended. Dress the broccoli with 1/3 cup of dressing and toss well. Chill the salad for one hour before serving.