In a blender, add the sugar, vinegars, onion, mustard, and salt. Cover and blend until smooth.
With the blender on low, slowly stream in the oil. When all of the oil has been added, stop and taste the dressing, adding more salt if necessary.
Stir in the poppy seeds. Serve on strawberry salad or your favorite green salad.
Notes
Store in an airtight container in the refrigerator for up to a week.To make this dressing without a blender, grate the onion instead of mincing it. Whisk all of the ingredients except for the oil and poppy seeds together until well combined. Whisk vigorously while slowly streaming the oil into the bowl. Stir in the poppy seeds.