In this recipe that I learned from Jean-Georges Vongerichten, horseradish (which loses most of its potency when heated) gives classic potato gratin a gentle kick in the ass. The trick is finding fresh horseradish—and then peeling and slicing it (some people wear goggles, not a terrible idea).

Makes: 4 servings
Time: 45 minutes

Ingredients

• 1 pound fresh hors…