In a large pot over high heat add about one third of the ground meat. Using a wooden spoon break up the meat and cook it until there are no lumps left and no traces of pink meat. Continue cooking the meat until all the liquid has evaporated and it starts to fry in its own fat and become nicely browned.
Transfer the meat to a bowl and repeat with the second and third batches of meat. Transfer them to the bowl also. Don't skimp on this step. Take your time to properly brown the meat which will add depth of flavor to the sauce.
While the meat is browning, add the optional pancetta along with the onion, carrot and celery to the work bowl of a food processor. Process the veggies to a paste and set aside. **See note below.
When all the meat has been browned, lower the heat to medium and add the puréed vegetables to the pot with about a teaspoon of salt. If there isn't enough fat rendered from the meat to cook the veggies, add about a TBS of olive oil.
Stir and cook the vegetables until they are soft and beginning to brown a little – about 5-8 minutes.
Add the minced garlic and cook for about 30 seconds until the garlic becomes fragrant.
Stir in the tomato paste and cook for a minute.
Add the wine and cook for a few minutes until it evaporates, scraping up the brown bits on the bottom of the pot. ***See note below.
Add the meat back to the pot along with the stock (or water), tomatoes, milk and another teaspoon of salt.
Bring the sauce to a boil, reduce the heat and let it simmer gently, uncovered for as long as you can – at least an hour and up to 4 hours – stirring occasionally, until it thickens. If it looks like it is getting too dry, add a dribble more of stock or water.
Check the sauce for seasoning and add pepper and more salt if needed. ****See note below.
In the meantime, put a large pot of well salted water on to boil.
Add the pasta, bring the pot back to a boil and cook to the package instructions.
About 2 minutes before the instructions say it is done, start testing the pasta for doneness; taste it! Using tongs or a fork, pick out a strand of pasta and bite it. If it is still very hard in the middle continue cooking another minute and taste again. Keep doing this until it is al dente; just cooked through but still with some resistance under your tooth.
Scoop out and save a cup or so of the pasta cooking water then drain the pasta into a colander.
Return the spaghetti to the now empty pot and add a ladle or two of the sauce, tossing to coat the pasta. (Adding the sauce right away will prevent the pasta from clumping together.) *****See note below.
If the sauce is too thick, add some of the reserved pasta water to thin it out a little.
Serve the pasta on warm plates with a generous scoop of sauce on top.
Sprinkle generously with parmesan cheese, passing more at the table. Use the good stuff, freshly grated. You've gone to all this trouble to make an excellent sauce and perfectly cooked pasta, you don't want to ruin it now by using inferior cheese.