Puree the onion in a blender until smooth. Strain through a fine mesh strainer. Reserve 3 tablespoons of onion juice; discard the remaining pulp.
Combine the sugar, mustard powder, and salt in the bowl of an electric mixer. Use the paddle attachment to mix the sugar until the spices are evenly blended. Pour in the reserved onion juice, soy sauce, and rice vinegar. Mix for several minutes until the sugar has begun to dissolve. Slowly drizzle in the canola oil with the mixer running at a medium-high speed. Add the toasted sesame seeds and continue mixing 1 minute more.