01 -
In a small bowl, whisk together potato starch (or cornstarch) with ¼ cup water. Set aside.
02 -
Roughly chop the onions and blend them with ¼ cup water until smooth.
03 -
Pour the blended onion mixture through a fine-mesh sieve into a bowl, pressing to extract as much liquid as possible. Discard any remaining pulp.
04 -
In a saucepan over low-medium heat, add the strained onion juice, sugar, white wine vinegar, poppy seeds, salt, and black pepper. Stir to combine.
05 -
Once the mixture is heated, whisk in the slurry and continue stirring. Bring the sauce to a quick boil, then immediately turn off the heat.
06 -
Let the sauce cool completely, as it will thicken further. Store in an airtight container in the refrigerator.