Heat cooking oil in a large pan over medium heat. Stir fry chicken until lightly browned and crisp.
In a small jug or bowl, whisk together soy sauce, sugar, sake, mirin, and sesame oil. Set aside.
Add the garlic to the center of the pan and sauté until lightly fragrant (about 30 seconds). Pour in the sauce and allow it to cook, while stirring, until the sauce thickens into a beautiful shiny glaze (about 2-3 minutes).
If the sauce needs thickening, whisk in the cornstarch/water mixture and continue stirring until syrupy.
Add in the steamed broccoli. Garnish with green onion slices and serve over steamed rice.