Preheat the broiler and place the peppers on the rack about 5 inches away from the source of heat. Broil peppers until the skins blister, turning them carefully so they cook on all sides. When the skins are blackend and cracked, 2 to 3 minutes on each side, transfer to a big bowl and cover with plastic wrap. Let them sit for about 10 minutes. The skins will detach completely, and the peppers will be very easy to peel. Peel the peppers, discard the stems and seeds, and cut the flesh lengthwise into 2 or 3 sections. Collect the juices.
Mix the oil and vinegar in a small glass jar. Add the pepper juices, garlic, mustard, chili pepper, and salt and mix well.
While the peppers are still warm, arrange them in a deep dish and pour the dressing over them. Partially cover the dish and let it cool, then cover and refrigerate until 1 or 2 hours before you are ready to serve. Serve at room temperature.