Meanwhile, make the relish. Heat the olive oil in a large frying pan, add the onion and chiles, and cook over medium heat for about 5 minutes until softened. Stir in the tomatoes, then season with salt and pepper and cook for 6–8 minutes until the tomatoes are beginning to collapse. Add the vinegar and stew down over medium heat for about 6 minutes until reduced to a rough relish consistency. Remove from the heat, stir in the basil, and season well with salt and pepper. Tip into a serving bowl and set aside.