Preheat oven to 425°F. Spray a 2-quart baking dish with nonstick cooking spray.
Spread the uncooked spaghetti and the red bell pepper in the bottom of the prepared dish.
In a separate bowl or large measuring cup, whisk together the chicken broth, soy sauce, hoisin sauce, sesame oil, brown sugar, garlic, ginger, and red pepper flakes. Pour over the pasta. Cover with foil and bake for 40 minutes, stirring halfway through.
Uncover; stir again. At this point, you should check the pasta to make sure that it is al dente (firm but just about finished cooking). If it’s still too hard, cover the dish and return to the oven until pasta is al dente. Then, move on to the next step.
Stir in the chicken and half of the green onions. Return the dish to the oven and bake, uncovered, for about 5 more minutes, until the chicken is warmed through and the pasta is tender. Give the pasta another stir, and then allow the dish to rest for a few minutes before serving.
Garnish with the remaining green onions, chopped peanuts, and/or toasted sesame seeds just before serving.