In a liquid measuring cup or wide-mouth jar, combine the aquafaba, mustard and 1 tablespoon of the vinegar. With an immersion blender running on high speed, drizzle the oil in a thin stream to blend and make the aquafaba “mayonnaise.”
In another wide-mouth jar, blend together the remaining 3 tablespoons vinegar and the sugar, lemon zest, lemon juice, miso, garlic, onion powder, celery seeds and salt until smooth. Return the blender to the aquafaba mayonnaise, and with the blender running at high speed, drizzle in the lemon-miso mixture. Stir in the poppy seeds, if desired. Taste and adjust with salt, vinegar and lemon juice as needed.
Cover and refrigerate remaining dressing for up to 1 week.
Tips: Aquafaba is the liquid left over from cooking beans and is often used for its thickening properties. The liquid from a can of chickpeas works well here.