Served hot or cold, either as a side dish or as part of a buffet… accompanied by plenty of crusty bread of course.
Enough for 6.

       1 large onion, thinly sliced
       1 large red pepper, thinly sliced
       1 large yellow pepper, thinly sliced
       3 tbsp olive oil
       2 chopped fresh tomatoes (or 300g canned chopped tomatoes)
       1 teaspoon wine vinegar
       1 teaspoon sugar
        salt and pepper

Heat the olive oil in a large saucepan over medium low heat.  Add the sliced onion and peppers, cover with a piece of greaseproof paper or a lid, and gently sweat for about 15 minutes (check that the vegetables are not browning, if they are lower the heat further).  Now add the chopped tomatoes and continue to cook uncovered for about 20 minutes. Remove from the heat, stir in the vinegar and sugar and season to taste. Serve lukewarm or cold.