Heat the oil in a frying pan and fry the onion, garlic and chilli until soft.
Add the tomatoes and mix well.
Add the vinegar and sugar and bring to the boil. Simmer for 30 minutes and season with the salt and freshly ground black pepper. The relish should be the consistency of jam.
Stir in the capers and gherkins and the chopped coriander. Add more seasoning if required.