Prepare the cornstarch slurry by whisking 1 tablespoon cornstarch with 2 tablespoons cold water in a small bowl until fully dissolved with no visible clumps. Set aside.
In a medium heavy-bottomed saucepan, combine soy sauce, brown sugar, mirin, and rice vinegar. Whisk to begin sugar dissolution.
Add minced garlic, grated ginger, and red pepper flakes (if using) to the saucepan.
Place over medium heat and bring to a simmer, stirring occasionally. Cook for 2 minutes to allow the aromatics to release their flavor compounds.
Reduce heat to medium-low and simmer for 5 minutes to begin reduction and flavor concentration.
Re-whisk the cornstarch slurry (it will have settled) and pour it into the simmering sauce while whisking continuously to prevent lumping.
Increase heat slightly and continue cooking, stirring frequently, until the sauce thickens noticeably and coats the back of a spoon, approximately 3-4 minutes. The ideal temperature for maximum thickening is 203°F (95°C).
Remove from heat and stir in sesame oil. The residual heat will integrate the oil without volatilizing its essential flavor compounds.
Allow to cool for 5 minutes before using. The glaze will continue to thicken slightly as it cools.