Peel and chop the onion. Put in a saucepan along with two tablespoonfuls of olive oil and soften over a low heat stirring from time to time.
Meanwhile peel and finely chop the garlic cloves, and top and tail the chilies, split lengthways, remove the seeds and finely chop. Add the garlic and chiles after five minutes or so. Continue until the onion is softish Picture .
Add the plum and sunblush tomatoes to the onion mixture and cook over a low/medium heat for about fifteen minutes. Stir every so often carefully so that most of the tomatoes are cooked but retain their shape Picture . When soft take off the heat and season to taste with salt and stir in some chopped oregano.
Carefully reheat when the dish that the salsa is accompanying is ready.