1Bring a large saucepan of generously salted water to a boil over high heat. Add broccoli rabe and cook until the stems just give when pierced with a sharp knife, about 2 to 3 minutes. Using a slotted spoon, remove broccoli rabe from the water and spread in a single layer on a baking sheet; set aside.
2Heat oil in a large frying pan over medium heat until shimmering. Add sausage in a single layer and brown on both sides, about 10 minutes total. Transfer to a plate and set aside.
3Add onion and 1/4 teaspoon salt to the pan and season with pepper. Cook, stirring occasionally, until onions are tender and browned, about 5 minutes. Add garlic and cook until fragrant, about 1 minute. Add chicken broth and bring to a boil, using a wooden spoon to scrape up any browned bits stuck to the bottom of the pan.
4Add reserved sausage and any accumulated juices to the pan. Transfer reserved broccoli rabe to the pan, leaving any excess water on the baking sheet. Cook, stirring occasionally, until heated through, about 2 minutes. Remove from heat, add lemon zest and juice, and stir to combine. Season with additional salt and pepper as needed.