Mix soy sauce, mirin, brown sugar, sesame oil, garlic, and ginger in a bowl to create the marinade.
Place the chicken breasts in the marinade, ensuring they are well coated.
Let the chicken marinate for at least 30 minutes.
Heat a skillet over medium heat and add a little oil.
Remove chicken from the marinade and cook for about 5-7 minutes on each side until golden brown.
In a separate bowl, mix cornstarch and water to create a slurry.
Add the remaining marinade to the skillet, then stir in the cornstarch slurry.
Cook until the sauce thickens, about 2-3 minutes.
Slice the cooked chicken and drizzle the sauce over it.
Garnish with sesame seeds and chopped green onions before serving.