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Put potatoes in saucepan of cold, salted water and bring to boil. Simmer 12-15 minutes, or until tender. Drain well, then leave in colander to steam dry and cool.
Meanwhile, mix vinegar and fine mustard to paste in large mixing bowl, then stir in sour cream until smooth. Stir in grainy mustard, followed by chopped shallot and celery. Season with sea salt and pepper, and refrigerate until potatoes have cooled.
Combine potatoes and mustard dressing and chill until a half hour before serving; potatoes should be served at room temperature.