Set sous vide machine to 54C/129F.
In a bowl, mix together soy sauce, sesame oil, ginger and garlic.
Put the flank steak in a freezer bag with the sauce mixture, and remove the air through a vacuum sealer or the displacement method. Drop beef in the bath for 2 hours.
Remove bag from bath. Take steak out of the bag, pat dry and lightly season with salt and pepper to taste.
Heat a large skillet on high and add oil. Sear for 1 to 2 minutes on each side. Remove from pan and place on plate to rest. Garnish with sesame seeds and green onions, serve and enjoy!