For making authentic Bolognese sauce, first make a purée of the carrot, the celery and the onion with a blender (called sofrito in Italian). If you don't have a strong blender, simply grate the vegetables.
Grate the pancetta, make this also look like a purée. If it's difficult to get Italian pancetta in your neighborhood, you can use bacon instead.
Heat the olive oil and the butter in a large pan. Add the vegetable purée and let the water content evaporate. It takes about 15 minutes to get the right consistency.
Add the ground pancetta, pork, and beef. Stir occasionally over medium heat, and let it simmer for 30 minutes until all liquids evaporate.
Pour in the wine and let it simmer for about 5 minutes.
Stir in the tomato purée. If you use tomato paste, dilute it first in the chicken stock. If you use tomato purée or canned tomatoes, you don't need the stock. If you prefer canned tomatoes, use 1 ½ cups for this recipe. Salt and pepper according to taste.
Cook your authentic Bolognese sauce for 90 minutes over medium low, stirring occasionally. Add some chicken stock or water to compensate for the evaporated liquids.
Pour in the milk little by little. It's important to use whole milk, some Italian recipes even call for half and half. I used cream and whole milk in 1:4 ratio. Let it come to a simmer once again, than turn off the heat and your authentic Bolognese sauces is ready.