Whisk together the water, low-sodium tamari, maple syrup, rice vinegar, red chili flakes and cornstarch until smooth and no cornstarch lumps remain. Set aside.
Heat a small frying pan over medium heat. Add a splash of olive oil, followed by the minced garlic and ginger. Cook for 1-2 minutes, stirring frequently, until fragrant. Be careful not to burn the garlic.
Give the sauce mixture another whisk to redistribute the cornstarch, then pour it into the pan. Bring to a gentle boil, reduce the heat to medium-low, and simmer for about 3 minutes, stirring until thickened.
Take off the heat and stir in the toasted sesame oil and stir in additional red chili flakes, sesame seeds, and green onion if using.
How to use this stir fry sauce: Cook your tofu, vegetables, or noodles first, then pour the prepared sauce into the pan toward the end of cooking. Toss everything together until evenly coated and heated through. If the sauce becomes too thick, add a splash of water until it reaches your preferred consistency.
Store leftovers in an airtight jar in the fridge for up to 1 week.