Preparation Cook potatoes in medium pot of boiling salted water until just tender, about 6 minutes. Drain, reserving 2 tablespoons cooking liquid. Whisk cream, mustard and reserved potato cooking liquid in large bowl to blend. Add potatoes to dressing and toss to coat. Refrigerate until slightly cooled, about 5 minutes. Mix ham, onion and celery into salad. Season salad to taste with salt and pepper. Arrange tomato slices on plates. Spoon salad atop tomatoes.
Preparation Cook potatoes in medium pot of boiling salted water until just tender, about 6 minutes. Drain, reserving 2 tablespoons cooking liquid. Whisk cream, mustard and reserved potato cooking liquid in large bowl to blend. Add potatoes to dressing and toss to coat. Refrigerate until slightly cooled, about 5 minutes. Mix ham, onion and celery into salad. Season salad to taste with salt and pepper. Arrange tomato slices on plates. Spoon salad atop tomatoes.