Make soffritto (mirepoix or Holy trinity) from carrot, celery and parsley root: cut the vegetables into smaller pieces and coarsely chop in a blender.
Briefly sauté onions and garlic in olive oil, add soffritto and sauté over a medium-high heat for 8-10 minutes, or until softened, stirring occasionally.
Stir in the ground beef and pancetta, turn the heat up to high and cook for 5 - 10 minutes, or until browned all over, stirring and breaking it up with a spoon.
Add the tomato purée, red wine, beef stock, bay leaves and thyme. Stir well, bring to the boil. Season with pepper, reduce to a low heat, cover and simmer for around 40 minutes, stirring occasionally. Remove the lid and continue cooking for about 15 minutes, or until thickened and reduced. Discard the bay leaves and thyme sprigs. Season with pepper.
Cook the spaghetti (al dente) according to packet instructions. Drain, but save about ½ cup of the cooking water.
Stir the spaghetti into the sauce, adding a splash of pasta water to loosen, if needed. Divide between bowls and sprinkle with Parmesan cheese, then serve.