Tear or cut the bread into chunks of around 3–4cm/1¼–1½in. Sprinkle with a little water, just to help moisten the surface. Add the water gradually – you don’t want it sodden, as it will also soak up some of the tomato juices.
Sprinkle the red onions with a pinch of salt and put in a bowl of cold water for half an hour, then drain and set aside.
Roughly chop the tomatoes and put in a bowl, along with any juices which may have escaped. Add the garlic and oregano and season well with salt and pepper.
Drizzle over the olive oil, then add the bread, onions, olives and basil leaves. Leave to stand for half an hour at room temperature before serving.