Mustard is, well, cutting the mustard. As more Americans brown-bag lunch and dine at home, sales of the sauce are up. Revenue at market leader French's grew more than 6% last year. But mustard's sometime partner and neme-sis is doing even better: This past winter, sales of Heinz ketchup rock-eted 9%. For a response, we called Barry Levenson, curator of the World Famous Mustard Museum in Wisconsin.