Tie the beef fillet with butchers string into a neat roast. Mix together the soy, brown sugar, rice vinegar, sesame oil, mirin and ginger. Place the beef fillet in a dish and cover with the marinade. Place in the refrigerator and marinade for at least two hours or overnight. Remove from the refrigerator 30 minutes prior to cooking, to stand at room temperature.
Set up your barbecue for indirect cooking. Please refer to your handbook for further instructions. Preheat your barbecue for roasting.
Spread the black sesame seeds on a tray or chopping board. Remove the fillet from the marinade, shake off excess marinade and roll in the black sesame seeds. Reserve the marinade.
Once the barbecue has preheated, make adjustments to the burner control(s) if required. Place the fillet on the barbecue and roast for 30 minutes.
While the beef is cooking, prepare the ponzu sauce. In a saucepan, combine the reserved marinade, orange juice and lime juice. On a side burner or stovetop, heat over medium heat until sauce has reduced and thickened slightly. Set aside to cool while the beef is roasting.
When the beef has finished cooking, remove from the barbecue and set aside to rest for 15 minutes. Once rested, thinly slice the beef, pour half of the ponzu sauce over the beef and garnish with the micro herbs.