Make dressing: Purée dressing ingredients in a blender until smooth, about 2 minutes, then transfer to a large bowl. Make salad: Cook pasta in a 6- to 8-quart pot of boiling salted water until tender. Drain in a colander, then rinse well under cold water. Add pasta, scallions, bell peppers, and sesame seeds to dressing, tossing to combine, and serve immediately.