To make the sauce:
Puree the onions, carrots, celery and garlic to a coarse paste in a food processor. Coat a large wide pan with olive oil and heat over medium heat. Add the pancetta and cook until it's brown and crispy. Add the pureed veggies and cook them until they become very soft and aromatic and are starting to turn brown, about 8 to 10 minutes. Stir in the white wine and let it reduce by half. Add the tomatoes, chicken stock, bay leaves, and thyme bundle and season with salt, to taste. Bring to a boil, then reduce heat and simmer for 15 minutes. Reserve.
To prep the cabbage:
Add the vinegar to large pot of well-salted water and bring it to a boil over medium heat. Set up a bowl of well-salted iced water. Add the cabbage to the boiling water and cook for 3 to 4 minutes, then immediately plunge it into the bowl of iced water. When the leaves are cool, remove them from the water and pat them dry. Reserve.
Coat a large saute pan with olive oil and put over medium heat. Add the onions and season with salt, to taste. Cook the onions until they are translucent and very aromatic, about 6 to 7 minutes. Add the garlic and cook for another 2 to 3 minutes. Turn off the heat and let cool.
In a large bowl combine the ground pork, pork sausage, chicken livers, if using, bread crumbs, Parmesan, egg, rosemary and crushed red pepper flakes. Season with salt, to taste. Add the chicken stock and mix well to combine. Stir in the cooked onions and garlic. Make a tester patty, cook it and eat it. It should taste really good, if it doesn't, adjust the seasoning.
Lay each cabbage leaf on a flat work surface. Spoon a generous 1/4 cup of filling into each leaf. Fold the outside edges in and roll the leaf around the stuffing. Arrange each roll with the seam side down so that the roll will hold itself shut.
Add the cabbage rolls to the sauce. Cover and simmer over low heat for 15 minutes. Uncover and simmer until the sauce has reduced to a thick, sauce-like consistency, about15 to 20 minutes. Taste and adjust seasoning, if necessary. Transfer the cabbage rolls to a serving platter and lightly cover with the sauce. Sprinkle with Parmesan and serve.