Heat half of the oil in a large pot until hot. Add the vegetables and saute until softened but not browned (about 7 minutes).
Add the beef, pork, and pancetta. Cook, breaking up the meat with a spatula, until browned.
Add the white wine and reduce by half.
Add the tomato sauce and chicken stock. Stir to combine. Cover the pot and simmer on low for 2 hours.
Meanwhile, cut the spaghetti squash in half lengthwise and scoop out the seeds. Preheat the oven to 400°F.
Brush the fleshy part of the squash with remaining oil. Place it on a baking sheet, cut side down.
Roast for 30 to 40 minutes or until fork tender and lightly browned on the outside, but still a bit firm. The time will vary depending on the size of the squash.
Remove the squash from the oven, turn it face up and allow it to cool until you can shred it with a fork.
Back to the sauce, remove the cover of the pot and add the milk. Simmer the sauce uncovered for 1 to 2 more hours.
Season with salt and pepper, to taste.
Serve the shredded spaghetti squash with the Bolognese sauce.