Preheat oven to 400°F.
In a large bowl, add soy sauce, brown sugar, garlic, and red pepper flakes and stir to combine. Set aside.
Cut chicken thighs into bite size pieces, roughly chop sugar snap peas, slice red bell peppers into thin strips and add all to the bowl with the sauce.
Slice scallions into small pieces, and add the white parts to the bowl, reserving the green parts for garnish at the end.
Toss sauce, chicken, and vegetables together until coated.
Generously spray a sheet pan with non-stick cooking spray, and add chicken mixture to the pan, spreading into an even layer.
Bake for about 20 minutes, stirring half-way through, until a meat thermometer inserted into the largest piece of chicken reads 165°F.
Turn the oven on to Broil and broil chicken for another 5 minutes or until the chicken and vegetables take on a little bit of browning.
Divide into four servings. Optionally, you can serve over rice or noodles.