Can be served with all cold meats.
Remove stems and seeds from the peppers. Slice the peppers lengthwise. Place the peppers, tomatoes, onions, olive oil and salt in a saucepan. Cover and simmer 1 hour, stirring from time to time.
Add the vinegar and continue to simmer, uncovered, until most of the liquid has evaporated. Cool and place in covered bowl or jar. Refrigerate for two days before using.