Using a small sauce bowl, mix corn starch with water. Whisk until corn starch has fully dissolved in water.
Add the rest of liquid ingredients wuch as the soy sauce, sweetener of choice, ketchup, and rice vinegar. Mix until well combined.
Heat a small sauce pan in medium low heat. Drizzle with a coat of cooking oil, no more than a teaspoon. Once fully heated, sauté onions, garlic, and chili flakes. Sauté until fragrant and onions have turned transparent. Careful not to burn garlic or red pepper flakes.
Pour liquid ingredients in sauce pan. Whisk until thicken and to ensure sauce does not stick (or burn) on the bottom. This should take between 3-7 minutes depening on your stovetop. Turn off heat. Set sauce aside or transfer back to sauce bowl to set aside.