1. Preheat an 8-inch nonstick skillet over medium heat. Add the walnuts and cook, stirring frequently, for 1 to 2 minutes, or until lightly toasted. Transfer the nuts to a plate to cool.
2. Place the broth, vinegar, honey, cayenne pepper, and salt, if desired, in a small bowl, and stir to mix. Set aside.
3. Bring a 4-quart pot of water to a rolling boil. Add the pasta, and cook according to package directions.
4. While the water is heating up, coat a 12-inch nonstick skillet with cooking spray, add the oil, and preheat over medium heat. Add the garlic and cook, stirring often, for 2 minutes, or until soft but not brown.
5. Increase the heat to high, and add the red bell peppers and half of the broth mixture to the skillet. Bring to a boil; then reduce the heat to low, cover, and simmer for 10 minutes. Stir in the scallions and the remaining broth mixture, and continue to simmer for 2 to 3 minutes, or until the peppers are just soft.
6. Drain the pasta, and add it to the skillet along with the toasted walnuts. Toss to mix well.
7. Spoon the pasta into individual serving dishes, and serve immediately.