In a small bowl, create a slurry by whisking together the cornstarch and 1/4 cup of the water until smooth. This is crucial to prevent lumps.
Add both the creamy chicken and shrimp seasoning packets to the slurry. Whisk until the powders are dissolved.
Add the remaining 1/4 cup of water, the soy sauce/oyster sauce, and sesame oil. Mix everything together.
If using, stir in the sugar and garlic.
Pour the mixture into a small saucepan or skillet over medium heat.
Bring the sauce to a gentle simmer, whisking continuously. It will begin to thicken as the cornstarch activates.
Once the sauce has reached your desired consistency, remove it from the heat.
Taste the sauce and adjust as needed. If it's too thick, add a little more water. If it needs more umami or saltiness, add a bit more soy sauce or oyster sauce.