Put the oil, butter, and chopped onion in the pot, and turn the heat on to medium. Cook and stir the onion until it has become translucent, then add the chopped celery and carrot. Cook for about 2 minutes, stirring the vegetables to coat them well.
Add the ground beef, a large pinch of salt, and a few grindings of pepper. Crumble the meat with a fork, stir well, and cook until the beef has lost its raw, red color.
Add the milk and let it simmer gently, stirring frequently, until it has bubbled away completely. Add a tiny grating—about 1 teaspoon—of nutmeg, and stir.
Add the wine, let it simmer until it has evaporated, then add the tomatoes and stir thoroughly to coat all ingredients well. When the tomatoes begin to bubble, turn the heat down so that the sauce cooks at the laziest of simmers, with just an intermittent bubble breaking through to the surface. Cook, uncovered, for 3 hours or more, stirring from time to time. While the sauce is cooking, you are likely to find that it begins to dry out and the fat separates from the meat. To keep it from sticking, continue the cooking, adding /2 cup of water whenever necessary. At the end, however, no water at all must be left and the fat must separate from the sauce. Taste and correct for salt.
Toss with cooked drained pasta, adding the tablespoon of butter, and serve with freshly grated Parmesan on the side.
Ahead-of-time note: If you cannot watch the sauce for a 3- to 4-hour stretch, you can turn off the heat whenever you need to leave, and resume cooking later on, as long as you complete the sauce within the same day. Once done, you can refrigerate the sauce in a tightly sealed container for 3 days, or you can freeze it. Before tossing with pasta, reheat it, letting it simmer for 15 minutes and stirring it once or twice.
Variation of Ragù with Pork: Pork is an important part of Bologna's culture, its economy, and the cuisine, and many cooks add some pork to make their ragù tastier. Use 1 part ground pork, preferably from the neck or Boston butt, to 2 parts beef, and make the meat sauce exactly as described in the basic recipe above.
Recipe by Marcella Hazan. Copyright © 1992 by Marcella Hazan.