Add ¼ cup brown sugar, ½ cup low-sodium soy sauce, ¼ cup mirin or rice vinegar, ¼ cup water, 1 tablespoon honey, 2 cloves minced garlic (or 2 teaspoons minced garlic), 1 teaspoon grated fresh ginger (or ¼ teaspoon ground ginger), 1 tablespoon cornstarch, and ¼ teaspoon black pepper, if using, to a saucepan. Whisk until well combined.
Heat the saucepan over medium-high heat and bring the sauce to a boil. Reduce the heat to low and simmer for 3 to 5 minutes, stirring frequently, until the sauce has thickened.
Remove from heat and let cool slightly.
Serve immediately or refrigerate in an airtight container for up to 1–2 weeks.
