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.