In a blender or food processor, combine sugar, ground mustard, salt, rice vinegar, and onion. With the motor on, slowly add vegetable oil in a slow, steady stream; whirling until dressing is smooth and has a creamy texture. Stir in the poppy seeds.
Store, covered, in the refrigerator.
Dressing will keep approximately 7 to 10 days. Serve at room temperature.
Yields approximately 3 cups dressing.
Poppy Seed Salad Dressing Recipe: https://whatscookingamerica.net/salad/poppyseeddressing.htm