Roast the chiles and bell pepper by placing them under a hot broiler or on a hot grill or on a hot comal. Turn them frequently until all the skin is charred. (See How to Roast Chiles for more details)
While the chiles are blackening, slice the onion into 3-4 large slices (discard the stem and bottom piece) and place under the broiler or grill until browned (not blackened.)
Remove the charred skins from the chiles and pepper as well as the seeds and the entire stem.
Place in a food processor with the rest of the ingredients and pulse until the ingredients are finely chopped and well incorporated.