Toast the sesame seeds in a dry frying pan until they are golden brown. Pour the seeds into a mortar and pound with a pestle until the seeds start to release their oils.
Add the rice wine vinegar and soy sauce to the mortar and continue to pound. Slowly add the grapeseed oil and mix until slightly emulsified. Store in a glass jar with an airtight lid until needed. It will keep in the fridge for a month.
Jimmy open your oysters and drizzle a little dressing over each one. Eat just as they are or heat them in the fire for a few minutes first.