Shell beansbring a sweetness to pasta tossed with sausage.
Place bay leaves and peppercorns in a piece of cheesecloth, tie with a piece of kitchen twine, and place in a medium saucepan; add the beans, onion, and salt. Cover with cold water, and bring to a boil over medium-high heat. Reduce heat to low, and simmer until beans are just tender, 15 to 20 minutes. Remove from heat, and let beans stand in the liquid until cool. Drain, reserving liquid. Set aside. Discard cheesecloth and onion.
Bring a large pot of salted water to a boil for pasta. Pour olive oil into a large skillet, and place over medium-high heat. Add the sausages, and cook, stirring with a wooden spoon to break up the large pieces, until well browned, 8 to 10 minutes. Add the leeks, garlic, and thyme; cook, stirring frequently, until softened, about 4 minutes. Reduce heat to low; add beans and 1 cup reserved cooking liquid. Bring to a simmer.
Cook the pasta until al dente, about 9 minutes (meanwhile, cover the skillet with the sausage mixture, and keep at a low simmer). Drain in a colander, and transfer to the sausage-and-bean mixture. Add the parsley, season with salt and pepper, and toss to combine. Serve, topped with grated Parmesan cheese, if desired.