- Mix together sugar, mustard, salt, and vinegar in a medium bowl. Add onion juice and stir until combined. Add oil slowly, beating constantly with an electric beater, and continuing to beat until thick. (Helen advises: "When you think the mixture is thick enough, beat 5 minutes longer.) Stir in poppy seeds. Store in refrigerator.
*To make onion juice, place a medium onion in a mini chopper or blender and puree. Strain puree through a fine-mesh colander set over a small bowl to catch the onion juice.