Bring a large pot of salted water to boil, adding the fennel, 2 unpeeled garlic cloves, and salt. Blanch until the fennel is tender, then remove it with the garlic using a slotted spoon.
Peel the garlic and puree it in a food processor with fennel, adding a little water to smooth the puree. Set it aside, reserving the boiling water for the pasta.
Heat a large skillet over high heat. Add a tablespoon of oil, then the pancetta, shallots, and crushed pepper. Give the pan a swirl, then turn the heat to medium. Brown the pancetta, then add half the remaining garlic. Turn down the heat and stir frequently until toasted.
Add the pasta to the boiling water, stirring occasionally to keep the rigatoni from clumping. Cook for 10 to 12 minutes, until al dente.
When the pasta is three-fourths done, finish the sauce. Add the fennel puree, butter, and 1 tablespoon of olive oil to the skillet with pancetta. If you’re using raw clams, add them now (or with steamed littlenecks, add once the pasta is done). Season the sauce lightly with salt, then raise the heat and cook until the sauce is bubbling and reduces slightly. Add pasta water as necessary to keep the sauce loose; you want it moist enough to coat the pasta.
Throw in the parsley leaves and toss again. Chop the remaining garlic and add to the sauce with lemon zest. Taste for seasoning and adjust with salt.
Spoon the rigatoni onto warm plates. Top each portion with bread crumbs, season with pepper, drizzle with olive oil, and serve.