Place the poblano chiles on a tray under the broiler, directly on a grill or directly on an open flame. I prefer to broil them, it’s faster and easier. Whatever method you choose, turn them every 2 to 3 minutes for a total of 6 to 9 minutes. They’ll seem charred and blistered on the outside; the flesh must be cooked but not burnt — like roasting marshmallows over a fire.
Once charred and hot, place them in a plastic bag, close it tightly and let them sweat for 10 to 20 minutes. Then, and preferably under a thin stream of cold water, remove the charred skin, make a slit down one side of the pepper and remove the cluster of seeds and veins. Cut the stem off and make slices that are about a 1/2″ inch wide.
Heat the oil and butter in a sauté pan over medium heat. Add the onion, cook for 6 to 8 minutes. Add the rajas (the poblano pepper strips) and cook for a few more minutes. Add the vinegar, cook another minute. Season with salt and pepper to taste.