In a small bowl, combine the chiles julienne and the vinegar and leave to soak for 1 hour. Meanwhile, in a small heavy skillet heat the oil over medium heat. Add the onion and saute, stirring occasionally, for 6 to 7 minutes, or until translucent. Add the garlic and the cumin and cook for about 2 minutes more, until the garlic has released its aroma. Remove from the heat and cool to room temperature. In a large bowl, combine the vinegar and chile mixture, the salt, pepper, and egg yolks. With a balloon whisk, whisk together until completely smooth, then continue whisking as you drizzle in the olive oil, very slowly at first, whisking until all the oil is absorbed and the mayonnaise is emulsified. Transfer to a bowl and stir in the onion-garlic mixture. Cover and refrigerate if not using right away, for up to 1 week.
c.1997, M.S. Milliken & S. Feniger, all rights reserved