Teriyaki sauce can be made ahead or while you're making the rice.
In a medium saucepan over medium-high heat, whisk soy sauce, sugar, vinegar, and fresh ginger. Heat to a low rolling boil. Stir as it heats.
In a small, separate bowl, combine cornstarch and water. Whisk with a fork until lumps are gone.
Add cornstarch and water mixture to boiling soy sauce mixture. (Don't add until it's boiling or it won't thicken.) Whisk together.
Turn heat down to medium-low and let it simmer for 7-9 minutes, stirring constantly, until thickens.
To serve, scoop rice and veggies onto a plate or bowl. Slice chicken on top and pour teriyaki sauce over the top. Top with sesame seeds and/or green onions, if desired. Enjoy!