Preheat your oven to 350F with the rack in the middle position. Prepare a 9"x13" baking dish with non-stick spray.
In a medium sauce pan (off the heat), whisk together the corn starch and the water to obtain a slurry. Whish in the soy sauce, brown sugar, sesame oil, ginger, and garlic.
Place the sauce pan on medium heat and bring to a simmer while whisking continuously. When the sauce thickens, continue whisking and cooking for a minutes. Take the Teriyaki sauce off of the heat and transfer in 2 dishes. Two thirds of the sauce will be used for brushing on the chicken. The rest can be reserved to serve with the cooked chicken (and rice). *Do not serve any of the sauce that has been in contact with the raw chicken.*
Place the chicken thighs in the prepared baking dish, skin side up. Brush the top generously with the sauce. Flip them, and brush them with more sauce. The thighs should be facing skin side down when going into the preheated oven. Bake for a total of 48 minutes, basting with more sauce, at 12 minute intervals.
After the second 12 minute interval (at the 24 minute mark) remove from the oven and flip the thighs so that they are skin side up and baste with more sauce. Place back into the oven for 12 minutes.
Take out of the oven and brush a final coat of Teriyaki sauce and bake for 12 minutes, or until the internal temperature is at least 165F.
When ready, you may want to place under the broiler to get more caramelization on the sauce. Make sure the baking dish is on the medium rack when broiling, so that the sauce does not burn. Keep an eye on it, it does not take very long.
Take out of the oven, and sprinkle with the sliced green onions and sesame seeds. Serve with rice and your veggies of choice. Enjoy!