1. Bring large pot of salted water to a boil. Add pasta and cook to al dente, according to package directions, stirring often. Reserve ½ cup of pasta cooking water, then drain pasta and set aside.
2. In large heavy pan or skillet, cook sausage over medium heat, breaking up with wooden spoon until lightly browned, 5 minutes.
3. Add oil and onion to skillet and cook until onion is softened, 3 minutes more. Add mustard greens and sauté just until wilted, 1 minute.
4. Add beans and pasta to pan along with reserved pasta water, tossing everything together and cooking until water has been absorbed, 1 to 2 minutes. Add salt to taste, drizzle with oil, and serve immediately.