Step 1If the pork is on the bone, cut it into pieces. If it is boneless, cut into large chunks (about 1 1/2 inches) that are about the same size. Season the pork all over with salt and pepper to taste and parsley.
Step 2In a meat grinder or food processor, chop together the onions, garlic and pancetta until they are finely minced but not pasty.
Step 3Heat the olive oil in a large casserole, preferably earthenware or cast iron, over low heat. Add the pancetta mixture and stir to combine. Scatter the pork pieces over the pancetta mixture, cover and cook very slowly. After about 30 minutes, give the mixture a stir, replace the lid and continue cooking until the onions have begun to color, about 30 minutes more. The meat won't brown.
Step 4Add the red wine and increase the heat so the liquid just comes to a simmer. Cook uncovered, stirring occasionally, until most of the liquid has evaporated, about 2 hours.
Step 5Raise the heat to medium, add the tomato paste, half at a time, and cook, stirring constantly, until the paste mixes in and becomes dark brown. Season to taste with salt.
Step 6Add the chopped tomatoes and one-fourth cup water, reduce heat to low, cover and cook another hour, stirring occasionally and adding water from time to time to keep the sauce somewhat liquid. Keep an eye on it; if the sauce gets too thick, it will tend to scorch toward the center of the pan. Just add a little more water and keep cooking. You may add as much as three-fourths cup in all, depending on the heat.
Step 7When the pork is tender enough that it starts to shred when stirred, after about 30 minutes, add the sausage and continue cooking the sauce for another 30 minutes or more. Season with salt and pepper to taste. The sauce should be very dark red, shiny and thick, almost sticky. If the meat has slipped from the bones, remove the bones before serving.