Steam the broccoli and cauliflower for about 3 minutes or until crisp cooked.
Refresh under cold running water; drain well.
Combine with the gherkins and tomatoes in a large bowl.
Make the dressing.
Combine the orange juice, mustard and tarragon in a bowl.
Gradually whisk in the oil.
Add salt and pepper to taste.
Pour the dressing over the vegetables; toss well to coat.