Peel and coarsely grate the celery. Halve melon and remove seeds. Unscrew the flesh with a ball cutter. Sort lamb's lettuce, wash and drain well. Chop tarragon leaves. Mix sour cream, oil, milk and lemon juice.
Add tarragon. Season sauce with salt, pepper and sugar. Mix celery, melon and salad. Arrange on a plate. Pour sauce over it. Serve the ham on top.