Toast the white sesame seeds in a large pan placed over high heat until the sesame seeds turn golden brown, which should take 4-6 minutes.
Then, grind the sesame seeds in a mortar and pestle (or with a food processor) until sandy.
Pour the ground sesame seeds into a bowl, add the mayonnaise and mustard to it, and mix it with a spoon.
Then, add the rice vinegar, light soy sauce, sesame oil, and sugar, and stir until evenly mixed.
You can use the dressing immediately to dress salads and stir-fries, and store the remaining in a jar or airtight container. It keeps well in the fridge for up to 3 weeks.
