Combine everything but the chicken and optional green onions in a small bowl. (See note.) Whisk to dissolve brown sugar. Place marinade and chicken in a zip-lock bag. Pop that bag in the fridge for a minimum of 4 hours, or longer if you have it (even overnight).
Preheat a grill (or an indoor grill pan or broiler). Spray cooking surface with cooking spray and place chicken on it to cook. Cook for 5-10 minutes on each side depending on the thickness. Using a digital thermometer, cook chicken until it is a minimum temperature of 165°. However, you’ll want to aim for between 170-175° for improved flavor and texture.
Optional: garnish with sliced green onions.
Notes
I recommend making 1 1/2 times the marinade. I reserve some of the marinade before adding the chicken and then use it in the last few minutes of grilling, or just brush it on after for extra flavor.