To begin, use a kitchen knife to finely chop the celery, carrots and onion. Cut them equally in size to allow for even cooking.
In a large pot, add the extra virgin olive oil. Warm on the stove over medium heat.
Add the celery, carrots and onion to the pot. Cook over low heat until soft (approximately 12 minutes), occasionally stirring with a wooden spoon. If you use the rosemary, add it here and stir for a further minute.
Add the pancetta and cook until the fat is released. Add tomato paste and cook, mixing until fully incorporated with the pancetta, for about 2-3 minutes.
Add the veal and beef, sauté, breaking up the ground meat with the back of a wooden spoon. Stir continuously until browned, for about 15 minutes.
Add the wine and stir until it evaporates completely.
Scrap any browned bits from the pot and add the San Marzano crushed tomatoes.Stir to combine, then add 2 1/2 cups of the chicken or beef stock and stir.
Reduce the heat to low and simmer, occasionally stirring for 2 hours. If the meat appears to dry out during this time, add a little bit of broth or water. Once cooked, season with salt and pepper.
In a small saucepan, bring the milk to a simmer then gradually add to the meat sauce.
Cover and simmer over low heat for 45 minutes, stirring occasionally. If the sauce is too thick, add 1/4 cup of stock to thin the sauce if needed. Taste again for salt and set aside.