Real porchetta takes three to four days to prepare. This ragu is a fun and fairly fast fix if you're in the mood for porchetta on the fly.
Preheat the oven to 350°. On a baking sheet, toss the breadcrumbs and butter; season with salt. Bake, stirring once, until toasted and golden brown, 12 to 15 minutes. Let cool.
Meanwhile, bring a large pot of water to a boil for the pasta.
In a large skillet or Dutch oven, heat the oil, two turns of the pan, over medium-high. Add the pancetta. Cook, stirring often, until the fat begins to render, about 2 minutes. Add the ground pork. Cook, stirring occasionally and breaking up the meat with a spoon, until lightly browned, about 5 minutes; season with salt and pepper. Add the rosemary, garlic, 2 tsp. lemon zest, the fennel seeds, crushed red pepper, and the fennel pollen, if using. Stir until aromatic, 2 to 3 minutes. Add the wine. Cook, stirring often, until reduced by half, about 5 minutes. Add the stock and the Parm rind, if using. Reduce heat to low. Let the sauce simmer while you cook the pasta.
Salt the boiling water and add the pasta. Cook until 1 minute shy of the package directions. Scoop out about 3/4 cup of the pasta cooking water, then drain the pasta.
Discard the Parm rind from the sauce. Add a generous handful of grated Parm, the lemon juice, and the pasta to the sauce. Toss, adding the cooking water as needed if the pasta is too dry. Season with salt.
Toss the breadcrumbs with the remaining 2 tsp. lemon zest, the parsley, and a bit more Parm. Serve the pasta in shallow bowls. Top with the breadcrumbs.