Bring a pot of salted water to boil for the pasta. Meanwhile, heat the oil and add the onion and sauté until translucent. Add the pancetta and cook until the fat begins to brown, but be careful not to make it crispy. Add the fava beans and cook for about 2 more minutes. Salt and pepper to taste. Add the broth and simmer for about 5 minutes. (Note: if you are using large fava beans, you should boil them for about 10 minutes prior to adding them to the pancetta.) Cook the pasta until 1 minute less then al dente, drain it, and add it to the fava bean mixture. Cook together for about a minute. (Note: if you would rather not add the broth above, retain about 1/2 cup of the pasta water and add it now.) Serve immediately, with a sprinkle of Pecorino cheese.