Whisk together the cornstarch and cold water until well combined then set aside.
1 tablespoon cornstarch, 2 tablespoons cold water
In a small saucepan, whisk together the water, sugar, rice vinegar, sambal oelek, soy sauce, garlic, and ginger.
½ cup granulated sugar, ⅓ cup rice vinegar, ½ to 2 tablespoons sambal oelek, 1 tablespoon soy sauce, 2 cloves garlic, 1 teaspoon fresh ginger
Bring to a gentle boil over medium heat, then reduce to a simmer for 3–4 minutes, stirring often, until the sugar is fully dissolved.
Whisk the cornstarch slurry again, then slowly whisk it into the simmering sauce.
Simmer 30–60 seconds until glossy and thick enough to coat a spoon. Remove from heat and let it cool for 5–10 minutes to finish thickening.