01 - In a mixing bowl, whisk together soy sauce, mirin, sake, honey, brown sugar, rice vinegar, grated garlic, grated ginger, and toasted sesame oil until well combined.
02 - Add the chicken thighs to the marinade, turning to coat evenly on all sides. Cover and refrigerate for at least 30 minutes, or up to 4 hours for more intense flavor.
03 - Preheat a grill or grill pan to medium-high heat. Brush the grates with vegetable oil to prevent sticking.
04 - Remove the chicken from the marinade, reserving the leftover marinade. Grill the chicken thighs for 5 to 7 minutes per side, until fully cooked through with a nicely charred exterior.
05 - While the chicken grills, pour the reserved marinade into a small saucepan. Bring to a boil, then reduce heat and simmer for 3 to 4 minutes until the sauce thickens slightly.
06 - Brush the grilled chicken generously with the reduced sauce. Transfer to a serving platter, garnish with sesame seeds and sliced green onions, and serve immediately.