To blanch zucchini and broccoli, add the cut vegetables to a pot of boiling water and cook for 1 to 2 minutes. Drain and then immediately rinse under cold running water to stop the cooking process.
In large bowl, combine pasta, bell peppers, onion, zucchini and broccoli.
Make dressing: In medium skillet, heat oil over medium heat. Add scallions and garlic and cook, stirring often, until softened, about 1 minute. Stir in miso. Gradually stir in broth. When miso and broth are well blended, stir in parsley and red pepper flakes. Pour miso dressing over pasta mixture and serve.