In a large pot, preferably of terracotta or enameled cast iron, make a soffritto by gently sautéing the onion, carrot and celery in olive oil until the vegetables are very soft, taking care never to brown them. (It helps to add a pinch of salt, which draws out the vegetables' own liquids, as well as a spoonful of water from time to time. This both speeds up the softening process and helps avoid browning.)

Add the ground pork (or beef) to the pot, breaking it up with a wooden spoon and mixing to combine it well with the soffritto. Season with salt and pepper, and let is simmer gently, stirring almost constantly, until the meat has lost its raw look. Then add a splash of red wine and continue simmering, until the wine has evaporated.

Add the tomatoes, stir again. Then let it simmer for at least an hour, stirring only occasionally, until the meat is tender and the sauce has thickened and developed a deep flavor. (You can let it go longer if you like and it will only get better the longer it cooks.) If the sauce gets too thick before you think it's done, then just add a bit of water or broth as needed.