Finely grate the onion on a microplane or the small holes of a grater to yield 1 tbsp with its juices.
In a medium bowl, whisk together the white wine vinegar, granulated sugar, Dijon mustard, kosher salt, and the grated onion with juices until the sugar starts to dissolve, 30–60 seconds.
While whisking constantly, slowly drizzle in the vegetable oil until the dressing is emulsified and lightly thickened, 2–3 minutes.
Whisk in the poppy seeds until evenly distributed.
Let the dressing stand 10 minutes for the flavors to meld. Whisk or shake before using and refrigerate leftovers in a sealed jar for up to 1 week.