If using pork shoulder or butt, cut off any skin and discard, and then cut the meat off the bone into chunks about 1 inch or so. In a food processor, pulse the pork in batches, about five times for a few seconds each time; set aside.
Heat the olive oil in a large heavy-based pot over medium heat. Add the pancetta and onions and sauté, stirring occasionally, until browned, 12 to 15 minutes. Add the garlic and crushed fennel seeds and stir for about 2 minutes. Transfer to a large bowl; return the pot to the stove.
Increase the heat to medium high. In the same pot, add just enough of the ground pork to make one layer. If using packaged ground meat, don’t crumble it; instead, break it into pieces (about 1 inch) to brown. Season with salt and pepper. Brown the meat all over, stirring occasionally, 3 to 5 minutes. Transfer the first batch of meat to the bowl and sear the remaining pork in batches, if necessary; add more olive oil as needed.
Return all the seared pork and the onion mixture to the pot. Add the sugar, oregano, bay leaf, red pepper flackes, 2 tsp. salt, and 1-1/2 tsp. pepper, and stir to combine. Pour in the wine to deglaze the pot, stirring up any browned bits stuck to the bottom of the pan. Let the wine reduce by at least half. Add the tomato purée and tomato paste and bring the sauce to a boil (the mixture will be thick).
Add 1/2 cup water, reduce the heat, and simmer the sauce, stirring occasionally, for 1-1/2 to 2 hours. As it cooks, juices will evaporate; periodically add water 1/2 cup at a time, letting it reduce after each addition, adding 1-1/2 to 2 cups total. After 1-1/2 hours, the meat should be tender and the flavors melded. Remove and discard the bay leaf. Immediately before serving, whisk butter into the sauce and toss with the pasta. Serve sprinkled with the Parmigiano, if you like.
The finished sauce (without the butter or Parmigiano) can keep for a week, covered in the refrigerator, and it can also be frozen for up to a month. Defrost frozen ragù overnight in the refrigerator and then slowly bring it to a simmer, adding a little water or broth to prevent scorching.