Boil the beets until they are cooked through and can be easily pierced with a knife. Drain, and place in a serving bowl.
In a medium frying pan, heat together the sesame seeds, garlic, mustard, soy sauce, rice vinegar, and sesame oil. Whisk for a quick minute just until the garlic is cooked, then whisk in the grapeseed (or peanut) oil. Pour over the warm beets. Season with pepper.
Serve hot or at room temperature.