1. In a small, heavy, dry skillet, toast walnuts over medium heat, stirring, until fragrant, about 5 minutes. Transfer to a small bowl and let cool.
2. In a large pot of boiling salted water, cook broccoli rabe until bright green and still crisp, about 1 minute. Drain and refresh under cold running water. Coarsely chop.
3. In a large pot, combine oil, garlic, rosemary and crushed red pepper; cook, stirring, over medium heat, until fragrant, about 1 minute. Add broth (or water) and bring to a boil. Add pasta and broccoli rabe. Increase heat to medium-high and cook, uncovered, stirring occasionally, until pasta is al dente and most of the liquid is absorbed, 15 to 18 minutes. Adjust seasoning with salt and pepper.
4. Serve in shallow bowls, sprinkled with reserved walnuts.