In a slow cooker, combine brown sugar, soy sauce, ketchup, minced garlic, ground ginger, black pepper, and red pepper flakes. Stir until well mixed.
Add the chicken thighs to the slow cooker, ensuring they are well coated with the sauce.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is tender and fully cooked.
Once cooked, remove the chicken from the slow cooker and set aside.
In a small bowl, mix cornstarch and water to create a slurry. Stir this mixture into the sauce in the slow cooker.
Turn the slow cooker to high and let the sauce thicken for about 10-15 minutes, stirring occasionally.
Return the chicken to the slow cooker, coating it with the thickened sauce.
Serve hot, garnished with chopped green onions if desired.