Prepare the Teriyaki Sauce: In a medium-sized mixing bowl, combine the soy sauce, mirin, honey (or brown sugar), rice vinegar, minced garlic, and grated ginger. Whisk the ingredients together until the sauce is well mixed and the sweeteners are fully dissolved.
Marinate the Chicken: Place the boneless, skinless chicken thighs in a ziplock bag or a shallow dish. Pour half of the teriyaki sauce over the chicken, ensuring it is evenly coated. Reserve the other half of the sauce for basting later. Seal the bag or cover the dish and let the chicken marinate in the refrigerator for at least 30 minutes (for best results, marinate for 2-4 hours).
Preheat the Grill or Grill Pan: While the chicken is marinating, preheat your grill or grill pan over medium-high heat. If using an oven, preheat it to 425°F (220°C).
Cook the Chicken: Brush the grill grates or grill pan with vegetable oil to prevent sticking. Remove the chicken from the marinade and place it on the grill or grill pan. Cook the chicken for about 4-5 minutes per side or until the internal temperature reaches 165°F (74°C). If using an oven, place the chicken on a baking sheet lined with parchment paper and bake for approximately 20-25 minutes, flipping halfway through.
Baste the Chicken with Teriyaki Sauce: While the chicken is cooking, brush it occasionally with the reserved teriyaki sauce. This will add extra flavor and help create a shiny glaze on the chicken.
Garnish and Serve: Once the chicken is fully cooked and glazed, remove it from the heat. Let it rest for a few minutes before slicing. Garnish the teriyaki chicken thighs with sliced green onions and sesame seeds. Serve them hot over steamed rice or with your favorite side dishes.