Prepare Chicken: Cut the chicken into bite-sized pieces and season with salt, pepper, and mirin.
Prepare Green Onions: Cut the green onions into 3-4 cm pieces.
Make Sauce: In a bowl, mix together soy sauce, mirin, sugar, and water.
Grill Green Onions: Heat some oil in a pan over medium-low heat and grill the green onions.
Cook Chicken: Push the green onions to one side of the pan, then add the chicken and cook until golden brown on both sides.
Simmer with Sauce: Once the chicken is cooked through, pour in the sauce and simmer until slightly thickened.
Serve: Serve the rice in bowls, top with chicken and green onions, drizzle with remaining sauce, and sprinkle with sesame seeds.