In a small bowl, whisk together the cornstarch with 1 tablespoon of the water until dissolved. Set aside.
In a small saucepan, whisk together the sugar, ⅓ cup water, Sambal Oelek, vinegar, and garlic. Bring the mixture to a boil and boil just until the sugar is dissolved.
Once the sugar is dissolved, whisk in the cornstarch slurry, and cook until the mixture is thickened, whisking constantly. It should only take a minute.
Cool the sauce and then serve immediately or transfer to a storage container.
Notes
Storage: Store in the refrigerator for up to 2 weeks in an airtight container or jar. This sauce does not freeze well. If the sauce thickens up too much as it sits, simply add a splash of warm water and whisk well or microwave for 10-15 seconds and then whisk. Cornstarch: In place of cornstarch, use equal parts of tapioca starch or arrowroot starch.