This sauce has many versions that originated in the Emilia-Romagna province in Italy in the 18 h century. To me, the most authentic recipe comes from Marcella Hazen and that is the version I make.
Classic Bolognese Sauce
1 tablespoon olive oil
3 tablespoons butter plus 1 tablespoon for tossing with the pasta
1/2 cup chopped onion
1/2 cup chopped celery
1/2 cup chopped carrot
1 pound ground beef chuck, not too lean
Freshly ground black pepper
1 cup whole milk
1 cup dry white wine
1 1/2 cups canned imported Italian plum tomatoes, cut up, with their juice
For serving: cooked pasta and grated Parmesan cheese
Put the oil, 3 tablespoons of butter, and chopped onion in a heavy-bottomed pot and turn the heat to medium. Cook and stir until the onion is translucent. Add the celery and carrot and cook for about 2 minutes, stirring to coat the vegetables with fat.
Add the meat, a large pinch of salt, and some freshly ground pepper. Break the meat up with a fork, stir well, and cook until the meat has lost its raw color.
Add the milk and let it simmer gently, stirring frequently until it bubbles.
Add the wine and let it simmer. When the wine has evaporated, stir in the tomatoes.
(Cooking the meat in milk before adding the wine and tomatoes protects it from the acidic bite of the latter.) When they 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—she says more is better), stirring from time to time. If the sauce begins to dry out, add 1/2 cup of water whenever necessary to keep it from sticking. In the end, there should be no water left, and the fat must separate from the sauce. Taste for salt.
Toss with cooked, drained pasta and the remaining tablespoon of butter. Serve freshly grated cheese at the table.