I've been thinking for a while about what to write about ragu bolognese, and encountered several problems, namely, as is the case of so many "classic" dishes, there's no one correct way to prepare it. The only two requirements for the sauce that remain undisputed are that it should have a long simmering time (at least three hours) and that it should never, ever be served over spaghetti.
The idea behind this is not only rooted in tradition (in Bologna, the sauce is almost always served over tagliatelle), but also in practicality (consider that thick, heavy sauce will cling much easier to tagliatelle or pappardelle, rather than to thin spaghetti). The long cooking time is not only meant to properly marry the flavors, but was once a way of softening low-quality meat, namely beef, which would've most likely been taken from an aged working ox.
Apart from these general rules, what constitutes a proper bolognese is widely disputed, so much so that in 1982, the Bolognese chapter of the Accademia Italiana della Cucina registered an official recipe based on a rumored 38 years of research and investigation. Their recipe includes ground beef (specifically skirt steak) and pancetta, along with a soffritto, milk, and tomato paste (milk, in addition to the long cooking time, would help to break down the fibers in the tough beef). However, absent from the recipe are ingredients such as ground pork, fresh tomatoes, garlic, and herbs.
That being said, it's recognized that even in Bologna, where the sauce originated, these standards are not always recognized. Lidia Bastianich, who writes of the "Ricetta Antica," which contains milk, and the "Ricetta Tradizionale," which is tomato-based and dairy-free, acknowledges that both are common in modern Italy. Basically what I'm saying is that the variations are endless. In some recipes, you might even find dried porcini mushrooms, ground pork or veal, and often chicken livers, hearts, or kidneys, so feel free to use your imagination (not to mention your leftovers).
For my recipe, I break the rules just a bit (apologies to the Accademia). I like to add pork, and sometimes veal, as I think it rounds out the flavors a bit. I also happen to love heaping tablespoons of rosemary, and often add thyme or bay leaves. And I add garlic, because I add it to just about everything.
If I could make a few rules of my own however, I think I'd say that the most important thing is to cook the sauce for a long time, and if you don't have a long time, don't bother to make it. Also, chop the vegetables very, very finely. I've found that this, along with the milk, helps the meats to melt down into a fine textured, supremely silky sauce.
Serving it over hand-cut pasta also helps.
2 tbsp olive oil
1/2 cup yellow onion, diced very fine
1/2 cup stalk celery, diced very fine
1/2 cup carrot, grated
3 cloves garlic
1/4 lb pancetta
1 can tomato paste
1 tsp minced rosemary
3/4 lb ground beef (or veal, or a mixture of both)
1/2 lb ground pork
1 cup milk
1 cup dry white wine
1 large can crushed tomatoes
2 cups beef stock
Puree pancetta and garlic in a food processor until it forms a paste. Set aside.
Heat olive oil in a large saucepan over medium high heat. Add onion and celery and saute 2-3 minutes or until onion is translucent. Add in carrot and pancetta/garlic paste and cook for 1-2 minutes or until the pancetta has rendered a bit of fat. Push the vegetable mixture to the edge of the pan and add in the tomato paste. Cook for another minute or two to give the paste a chance to caramelize. Add in the thyme, rosemary, pork, and beef, and stir to combine with the vegatables. Cook, stirring often and breaking up the meat, until the meat is cooked through (it might be hard to tell because the tomato paste colors the mixture--it should take about 15 minutes). Add the milk to the pot and simmer until it has reduced to almost nothing. Add the wine, tomatoes, and broth and bring to a boil. Reduce heat and simmer, uncovered, for 3-4 hours (or ideally, all day). Season to taste with salt and pepper.