Preheat oven to 300 degrees F. Rub the peppers lightly with olive oil and place on a sheet pan in the oven. Turn the peppers now and then until the skin blisters. Place in an ice bath. When cool, peel the peppers and remove the seeds. Julienne into 1-inch strips. Peel and cut onion into very thin slices. Chop the basil roughly. Place the garlic in a mortar and crush along with the remaining olive oil. Place all the ingredients in a mixing bowl, add salt, pepper and vinegar and toss gently. Serve cold or at room temperature.