1) Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, about 8 to 10 minutes. Drain pasta reserving 250ml of the cooking liquid.
2) In a large, heavy frying pan warm the olive oil over medium-high heat. Add the sausage and onions. Using a wooden spoon break up the sausage into bite-sized pieces as it browns.
3) Continue cooking until the sausage is golden and the onions are tender. Add the beans and oregano cook for 2 more minutes. Add the pasta cooking liquid and stir, scrapping up any brown bits from the bottom of the pan. Add the mascarpone cheese and stir until it dissolved into a light sauce. Add the salt, pepper, and hot pasta.