In a large saucepan, cook the sausage, breaking it up, over medium-high heat until browned, about 10 minutes. Transfer to a paper-towel-lined plate; discard all but 1 tablespoon fat from the saucepan. Add the onion to the saucepan and cook until softened, about 3 minutes. Stir in the broccoli rabe and chicken broth and bring to a simmer; cook for 3 minutes. Stir in the sausage and beans and cook until heated through, about 2 minutes; season with salt and pepper.