Combine red chilies, garlic, sugar, vinegar, and water in a saucepan. Stir to combine.
Bring to a boil, then reduce the heat to low. Let it simmer for 5 minutes, occasionally stirring.
In a separate bowl, dissolve the cornstarch in 2 tablespoons of water, and add it in.
Let the sauce thicken slightly for about a minute, then turn off the heat. Taste the sauce and adjust as needed.
Notes
I prefer to use mild peppers such as red bell peppers or red Korean chili peppers for flavor and color, and add a touch of heat from red bird's eye chilies, red Fresno peppers or red jalapeño peppers.Choose the type of pepper based on your personal preference. Mince the peppers almost to a paste. You can also use a blender or food processor. Dried red pepper flakes, gochugaru (Korean chili powder), sambal oelek, or chili garlic paste can also be used.