Put the oil, butter, and chopped onion in a pot over medium heat. Cook, stirring, until the onion turns translucent. Add celery and carrot and cook about 2 minutes, stirring to coat.
Add the ground beef, a large pinch of salt, and a few grindings of pepper. Break up the meat with a fork and cook until it is no longer raw and red.
Pour in the milk and let it simmer gently, stirring often, until it has bubbled away completely. Grate in about 1/8 tsp nutmeg and stir.
Add the wine and simmer until it evaporates. Add the tomatoes and stir well. When the tomatoes begin to bubble, lower the heat so the sauce cooks at the laziest simmer, with only an occasional bubble.
Cook uncovered for 3 hours or more, stirring from time to time. If the sauce starts to dry and fat separates, keep it going and add 1/2 cup water when needed so it does not stick. Finish with no water left and fat separated from the sauce. Taste and adjust salt.
Toss with cooked, drained pasta, adding the extra tablespoon of butter. Serve with grated Parmigiano-Reggiano.
