Place chicken in a large bowl. Add soy sauce, mirin, and brown sugar. Massage to mix and set aside. Meanwhile, prepare the rest of the ingredients. Cut the vegetables, garlic, onion and ginger.
Heat 1 tablespoon vegetable oil in a pot over high heat. Add carrots and potatoes. Fry until lightly caramelized. Remove from the pot and set aside.
Pour remaining vegetable oil in the pot. Add onion, ginger, and garlic. Cook for 2 minutes until onion is translucent. Add chicken WITHOUT the marinade (set the liquid aside). Cook until browned and longer pink. Stir occasionally.
Add the marinade. Simmer for 1 minute. Pour water and cook with the lid on until chicken is tender about 25-30 minutes. Longer if using thicker cuts of chicken. Add back carrots and potatoes. Simmer until vegetables are just tender.
Add sliced mushrooms. Cook until softened. Pour cornstarch slurry. Simmer for another minute until sauce thickens. Stir lightly.
Taste the sauce and adjust salt, sugar, and pepper to taste. Garnish with chopped onion leaves. Serve with warm rice and enjoy!
Notes
Mirin - can be substituted with regular cooking wine. You may need to add a bit more sugar to make up for its lack of sweetness.
Cooking time - adjust depending on the size of chicken pieces. The bigger/thicker the cut, the longer the cooking time.
Soy sauce - can be substituted with light soy sauce/ low-sodium if preferred.
Mushroom - I use white button mushrooms. You can also use cremini mushrooms, shitake mushrooms, and portobello mushrooms.