Fill a medium soup pot with water and bring to a boil over high heat. Boil the noodles according to package directions, omitting salt, about 9 minutes or until al dente. Drain the noodles in a colander, then transfer noodles to a large mixing bowl.
In a medium mixing bowl, whisk together the peanut butter or Tahini, soy sauce, sesame oil, brown sugar, ginger, and crushed red pepper flakes. Whisk until smooth.
Toss the noodles with half the peanut sauce and chill, up to a day in advance.
When ready to serve, pour the remaining peanut mixture onto the spaghetti and toss to coat. Sprinkle with the remaining ingredients and enjoy immediately.