In a small bowl, whisk together the soy sauce, rice vinegar, sesame oil, brown sugar, garlic, ginger (if using), and chili flakes until the sugar dissolves.
Use right away as a marinade or sauce, or simmer in a small pan over medium heat for 2 to 3 minutes to thicken into a glaze.
For an even thicker, pourable teriyaki sauce, stir in a slurry of 1 teaspoon cornstarch mixed with 1 tablespoon water and simmer until glossy.
Notes
Makes about 1/3 cup. This is my go-to sauce for chicken stir fries, salmon bowls, rice bowls, tofu, shrimp, meatballs, and noodles.
Other add-ins: a drizzle of chili oil for extra heat and depth, or fresh grated ginger for warmth.
Store in an airtight jar in the fridge for up to 1 week. Use tamari or coconut aminos to keep it gluten-free, and maple syrup or honey in place of brown sugar.