Chef John's Chicken Teriyaki

Whisk soy sauce, sake, mirin, brown sugar, white parts of green onions, and ginger together in a saucepan; bring to a simmer and cook until sugar dissolves and marinade flavors combine, about 5 minutes. Pour marinade into a bowl and cool to room temperature. Pour 1/2 the marinade into a resealable plastic bag. Add chicken thighs, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator, 30 minutes to 1 hour. Set oven rack about 6 inches from the heat source and preheat the oven's broiler. Line a baking sheet with aluminum foil and grease with vegetable oil. Arrange chicken thighs on prepared baking sheet. Discard excess marinade. Broil chicken in the preheated oven for 5 minutes. Flip thighs and broil for 5 minutes more. Flip thighs again and brush each thigh with reserved marinade. Continue to broil, brushing with reserved marinade every 1 to 2 minutes, until cooked through, about 12 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Garnish with dark green parts of green onions.