1/4 cup each of whole-grain mustard and Dijon mustard
2 tablespoons cider vinegar
Salt and freshly ground pepper
Light a grill. Rub the chiles and onion with oil and grill until charred, 8 minutes. Peel and seed the chiles and cut into thin strips; chop the onion. Transfer the vegetables to a saucepan. Add the remaining ingredients and bring to a simmer over moderate heat, then serve.