Whisk mayonnaise and Dijon mustard together in a mixing bowl until smooth, about 30 seconds, to start the emulsification.
1 cup mayonnaise, 1 tablespoon Dijon mustard
In a small separate bowl, stir granulated sugar into apple cider vinegar until no crystals remain, about 1 minute.
3 tablespoons apple cider vinegar, 2 tablespoons granulated sugar
Slowly pour the vinegar and sugar mixture into the mayonnaise while whisking constantly to maintain a stable emulsion, continuing for about 1 minute.
3 tablespoons apple cider vinegar, 2 tablespoons granulated sugar, 1 cup mayonnaise, 1 tablespoon Dijon mustard
Stir in celery seed, salt, black pepper, and optionally heavy cream and lemon juice until uniform, about 20 seconds. Taste and adjust seasoning as desired.
1 teaspoon celery seed, 0.5 teaspoon salt, 0.25 teaspoon black pepper, 2 tablespoons heavy cream, 1 tablespoon lemon juice
Cover and chill the dressing in the refrigerator for at least 30 minutes before serving to thicken and allow flavors to meld.