Whisk all ingredients together in a small saucepan (off the heat!) until smooth and no more lumps of cornstarch are visible.
Bring to a boil, whisking constantly. Then lower the heat to a simmer and cook, stirring, until the sauce has thickened to your liking.
If the sauce is too thick, add a little more stock. If the sauce is too thin whisk a little cornstarch into cold stock or water, then whisk this into the sauce and simmer until thickened enough. Do NOT add dry cornstarch directly into the hot sauce!
Pour into a clean jar while hot and allow to cool before closing with a tight-fitting lid and storing in the fridge.
Notes
The sauce keeps in the fridge for about 2-3 days with the scallions added, about a week without.