Prepare grill. Secure each onion slice horizontally with a wooden pick and brush both sides with oil. Season onion slices with salt and pepper and grill with chiles and bell peppers on an oiled rack set 5 to 6 inches over glowing coals, turning them occasionally, until soft and charred, about 20 minutes. Wearing rubber gloves, peel, seed, and devein chiles. Peel bell peppers and discard seeds and ribs. Cut chiles and bell peppers into strips. Heat a dry comal or flat iron griddle over moderately low heat until hot and pan roast garlic, turning it occasionally to ensure even roasting, until browned and soft throughout, about 25 to 30 minutes. Discard garlic skin. In a small heavy skillet dry roast oregano over moderate heat until fragrant and beginning to brown, shaking skillet occasionally, about 2 minutes. In a bowl mash together oregano, garlic, and salt to taste. Add onion slices, separated into rings, chiles, and bell peppers and toss to combine well.
Serves 4

* available at Mexican markets.