In the meantime, make the teriyaki sauce. Add all of the ingredients to a saucepan- soy sauce, water, sesame oil, rice vinegar, sugar, garlic, ginger, cornstarch, black pepper, and chili flakes (optional). Heat over medium-high heat, stirring occasionally. When the sauce starts to boil, turn the heat to medium-low and start whisking constantly, simmering the sauce for 2-3 minutes until it thickens. Remove from the heat.
½ cup low-sodium soy sauce, 1 cup water, 2 tablespoon brown sugar, 2 tablespoon rice vinegar, 1 tablespoon toasted sesame oil, 3 garlic cloves, minced, 2 teaspoon minced ginger, 2 tablespoon corn starch, ⅛ teaspoon Freshly cracked black pepper, ¼ teaspoon chili flakes