Although it's been around since ancient times, horseradish is a little scary for most French diners, who prefer to have strong mustard up their noses. You're most likely to find it around Alsace, although specialist greengrocers anywhere in France will sell you the root (sent over from Germany) if you order it in advance.
The foreign section in the supermarché may have the sauce (usually creamed and not very strong) imported from the UK or Germany.
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