Add olive oil to a large soup pot or Dutch oven set over medium-high heat and cook sausage, stirring often and breaking apart with a wooden spoon, until well-browned (about 8 minutes). Remove sausage to a bowl.
Add onion and salt and cook for 5-8 minutes, stirring often, until onion is softened.
Add garlic and hot pepper flakes and cook for an additional 1 minute.
Add chicken broth and increase heat to high. Bring to a boil, add pasta, and cook for 7 minutes, stirring frequently.
Add broccoli rabe and continue to cook for an additional 5 minutes or until pasta is al dente.
Stir in cooked sausage, ¼ cup Parmesan cheese, lemon juice, and lemon zest.
Serve garnished with remaining parmesan and topped with chopped basil.