In a medium bowl, whisk together the soy sauce, mirin, brown sugar, honey, rice vinegar, sesame oil, minced garlic, and grated ginger until the sugar fully dissolves.
Pour exactly half of the sauce, about 1/2 cup (120 ml), into a separate clean bowl and refrigerate. This reserved half is your finishing glaze and must never touch raw chicken.
Pat the chicken thighs dry, season with salt and pepper, and place in the bowl with the remaining half of the sauce. Refrigerate for at least 30 minutes and up to 8 hours, flipping once halfway through.
Pour the reserved sauce into a small saucepan and bring to a gentle simmer over medium heat. Whisk in the cornstarch slurry and cook for 1 to 2 minutes until thickened enough to coat the back of a spoon.
Heat the grill to medium, around 375 to 400°F (190 to 200°C). Scrub the grates clean, then wipe them with an oiled paper towel held in tongs.
Remove the thighs from the marinade, letting excess drip off, and discard the used marinade. Lay the chicken on the grate and cook, covered, for 5 to 6 minutes per side, watching for flare-ups.
During the last 2 minutes, brush both sides generously with the thickened glaze. Cook until an instant-read thermometer registers 175°F (80°C) in the thickest part of the thigh.
Let the chicken rest for 5 minutes, then brush on any remaining glaze and scatter the sesame seeds and sliced green onions over the top. Slice against the grain if serving over rice.