Add the soy sauce, brown sugar, water, mirin, sesame oil, rice vinegar, chili-garlic sauce, ginger, garlic and pepper to a small saucepan. Bring the mixture to a simmer.
¾ cup low-sodium soy sauce, ¾ cup brown sugar, ¼ cup water, ¼ cup mirin, 2 tablespoons sesame oil, 1 tablespoon rice vinegar, 1-4 tablespoons chili-garlic sauce, 1 tablespoon minced fresh ginger, 4 cloves garlic, ½ teaspoon ground black pepper