Steam zucchini, summer squash, broccoli and similar vegetables just until just tender, about 6 minutes. Drain well. Transfer to medium bowl. Add mushrooms. (Add tomatoes near the end of the toss to keep them from getting squished too much) . Combine mustard and vinegar in bowl. Gradually whisk in oil. Add herbes and cumin. Mix dressing into vegetables. Season with salt and pepper. Cool to room temperature and serve.