Roughly chop the tomatoes then heat the oil in the casserole, then add the onion and garlic and let them gently cook for 5-6 minutes, until they are softened and pale gold in colour.
Now add the chopped tomatoes with about a third of the basil, torn into pieces. Add some salt and freshly milled black pepper, then all you do is give everything a good stir and let the tomatoes simmer on a very low heat, without a lid, for approximately 1½ hours – 1 ¾ hours or until almost all the liquid has evaporated and the tomatoes are reduced to a thick, jam-like consistency, stirring now and then.
Next use a hand blender to whizz the sauce, but not too much, and stir in the rest of the torn basil leaves, and serve on pasta, with some freshly grated Parmigiano Reggiano. When serving this sauce, it is a good idea to give the pasta 1 minute less cooking time than you usually would, then return it to the casserole after draining and give 1 more minute while you mix in the sauce.