In a large bowl, combine soy sauce, brown sugar, ginger, garlic, and black pepper. Add the chicken thighs and marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
Heat vegetable oil in a large pot over medium heat. Remove the chicken from the marinade (reserve the marinade) and sear the chicken thighs for about 5 minutes on each side until golden brown. Remove the chicken and set aside.
In the same pot, add the reserved marinade, chicken broth, water, sesame oil, and red pepper flakes. Bring to a simmer.
Return the chicken to the pot, cover, and let it simmer for 25-30 minutes, or until the chicken is cooked through and tender.
While the chicken is cooking, prepare the jasmine rice according to package instructions.
Once the chicken is done, remove it from the pot and let it rest for a few minutes before slicing.
Serve the sliced chicken over a bed of jasmine rice, ladle the garlic ginger broth over the top, and garnish with sliced green onions and fresh cilantro.