Heat the olive oil in a large pot and sauté the onion until nicely golden and a bit sticky. Add the garlic and when it smells great, add the tomatoes. Bring to the boil and season with salt and pepper. Tear in about 7 of the basil leaves and simmer for 5 minutes or so, squashing down most of the tomato lumps with a potato masher. Add the not water and simmer, covered, for about 20 minutes.