In the recipe, I suggest small dicing your mire pox (carrot, onions, celery), but it is not necessary. You can cut your vegetables larger for a more rustic sauce or blend it in a robot coupe if you want it more refined. Cut the vegetables however you want, but it is essential to be consistent with that size. The more consistent you are with your cuts the more evenly everything will cook.

Once the vegetables are cooked and you have deglazed with wine remove from the pan.  It is time to brown your meat.  The Maillard effect is the process in which a chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavor and is a super important part of making bolognese. Not adequately caramelized meat makes a good sauce but will lack a depth of flavor that makes bolognese great. Most of the time you are making bolognese, you can be doing other things in the kitchen like drinking wine or eating cheese, but once the meat starts browning, you should be scraping the bottom of the pan to make sure the meat browns but does not burn.

When the ground beef is browned, add milk and braise the meat.  The lactic acid tenderizes the protein and adds richness to the sauce, don't skip this step. Braise the ground beef and pork on low until the liquid is about 85 percent reduced, then add the vegetables, pancetta, and tomato.  I like to use San Marzano tomatoes and hand-crush them (you can mill or blend them depending on the texture you want).  They are slightly sweet and a little less acidic than other varieties, which helps make a balanced sauce.  At this point, add the bay leaves and pepper make sure the burner is on low, and let the bolognese simmers for an extended period. Stir occasionally to make sure it is not sticking to the bottom of the pan or burning. I usually let it braise for about two hours, but ultimately decide when it is done by taste and texture.

A note on seasoning your bolognese: don't wait until the end to add salt, season as you go. It would be best if you seasoned the mire pox as it cooks as well as the ground meat. Seasoning as you go will result in a more complex flavor, and you are less likely to over season it at the end. 

Pappardelle made from the egg yolk pasta dough or the whole wheat casarecci that Sfoglina Denver makes with the rustic red wheat berry both pair really well with the bolognese.   Finish the bolognese with a little bit of cream, parmesan, fresh rosemary, and a ladle of the pasta water.  An ounce or two of pasta water will help emulsify the sauce.