Kale-­Leek Gratin With Gruyère
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The secret to this crowd-pleasing gratin is its easy, classic cheese sauce. Known as sauce Mornay to generations of French chefs, it’s the same base used to make traditional mac n’ cheese, thickened with a roux (pronounced “roo”), a mix of butter and flour. The leeks and kale can be cooked the night before, covered, and refrigerated overnight; bring to room temperature while you make the cheese sauce.