1. Bring a large pot of salted water to boil. Add pasta and cook until just tender, about 10 minutes.
2. In a blender, combine garlic, ginger, soy sauce, vinegar, sesame oil, peanut butter and sugar; blend until smooth.
3. Drain pasta, rinse under cold running water and drain again. Toss with sauce in a large bowl, adding more water, 1 tsp. at a time, as necessary to moisten. Add chicken, broccoli, carrots and bell pepper and toss again. Sprinkle with scallions and serve immediately or cover and refrigerate for up to 1 day.