Put the chicken, sake, ginger, salt and pepper in large pot or Dutch oven and use your hands to massage the marinade into the chicken. Set aside to marinate for at least 10 minutes.
Put the soy sauce, brown sugar, gochujang, sesame oil, garlic and onions in a blender and mix until smooth.
Add the blended mixture and the chicken stock to the pot with the chicken and bring to a boil over high heat. Reduce the heat to medium-low and cook for 15 minutes.
Add the carrots to the chicken and cook on medium heat for an additional 10 minutes. Using a ladle, skim off any fat from the surface, turn the chicken pieces in the liquid, add the potatoes and cook until you can pierce the potatoes with a fork and the sauce is thickened, for 10 to 15 minutes. Skim off any remaining fat from the top. Add the lemon juice and stir.
Serve over white rice and garnish with scallions and sesame seeds.