Heat a large pot with 2 tablespoons of olive oil over low heat. Fold in the carrot, celery and onion and cook, stirring often for about 10 minutes, until the veggies have softened.
Heat another pan with the remaining 1 tablespoon of olive oil over medium heat, fold in the pancetta and stir fry for 5 minutes, then add in the beef and continue to cook, stirring often, until it's browned.
Transfer the beef and pancetta into the pot with the veggies, and bring the temperature to medium-high. Pour in the wine and allow to evaporate, about 5 minutes.
Pour the tomato sauce into the pot, and add in the bay leaf (if using). Reduce heat to very low, partially cover the pot with a lid, then slowly cook the ragu' for at least 2 hours.
Stir the sauce occasionally, and if you see it's drying out, pour in a bit of stock. During the last half an hour of cooking, pour in the milk a little at a time, until fully incorporated.
The sauce is ready when it reaches a dense and rich texture. If it's still too runny, let it cook a little more .
Serve with egg tagliatelle or short pasta, and freshly-grated Parmesan cheese on the side.