For making authentic Bolognese sauce, first make a fine mix, a purée of the carrot, the celery and the onion with a blender. If you don't have a strong blender, simply grate the vegetables.
Mince your 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 vegetables and let them evaporate their water content. It takes about 15 minutes the get the right consistency.
Add the minced pancetta, pork and beef. Stir occasionally over medium heat, and let it simmer for 30 minutes until all liquids evaporate.
Pour in your wine and let it simmer for about 5 minutes.
Stir in the tomato paste or 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 1/2 cups for this recipe. Salt and pepper according to taste.
Cook your authentic Bolognese sauce for 90 minutes in low-medium heat, stirring occasionally. Add some water or chicken stock to compensate for the evaporated liquids.
As for the last step in making real, authentic Bolognese sauce, pour in the milk little by little. It's important to use whole milk, some Italian recipes even call for find 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.