Season the chicken thighs in a bowl and set aside
Heat olive oil or any neutral cooking oil on medium-high heat in a nonstick skillet and sear the chicken thighs on each side for 3-4 minutes until nicely browned on each side
Once brown, remove the chicken from the pan and set it aside
Reduce the heat to medium and add more olive oil if needed and saute garlic until fragrant, about 30 seconds
Add mirin, soy sauce, vinegar, dark soy sauce, cornstarch, brown sugar, water, and mix
Allow sauce to simmer and reduce for one minute, then add back the chicken thighs to the sauce, the sauce is the right consistency when you are able to part the sauce with a rubber spatula and see a line in the pan
Spoon the teriyaki sauce over the chicken thighs and cook for another 2 minutes or until chicken registers at 165 degrees internal temperature
Remove from skillet and plate, spoon over more sauce on the chicken and garnish with sliced green onions