40gr scallions (chopped, white and light green parts)
2 tbsp fresh lemon juice
1 tsp sesame oil
1 tsp fresh ginger
½ cup sesame seeds (white and black combined or white only)
½ tsp corn starch
In a large zip-top bag, combine the soy sauce, chopped scallions, lemon juice, sesame oil, and fresh ginger. Swish everything around in the bag until it's well combined.
Add the tuna steaks, turning within the bag to coat them completely with the marinade. Press the excess air out of the bag, seal, and marinate in the refrigerator for about 20 minutes.
Preheat a countertop contact grill to "sear" or the highest temperature setting, or preheat an open grill pan over high heat.
Place the sesame seeds on a plate or a shallow dish. Remove the tuna steaks from the marinade bag, brushing the scallions off the steaks and reserving the marinade. One at a time, coat the steaks in sesame seeds on all sides, pressing the seeds into the steak so they'll stick.
Spray the grill plates lightly with non-stick spray and place the steaks on the grill, closing the cover so the top grill rests evenly on the steaks. Do not press down. Grill for about 3 minutes for a rare, pink interior. Grill longer if you prefer your tuna cooked through. Remove from the grill and keep warm.
If you are using an open grilling surface, carefully flip the tuna steaks with a pair of tongs after 3 minutes and cook for 3 minutes on the other side.
While the tuna cooks, pour the marinade into a small saucepan and bring to a boil. Add the corn starch, stirring with a whisk. Simmer for about 3 to 4 minutes, until the sauce thickens.
To serve, slice the tuna steaks on the diagonal into thin slices. Fan the slices on a plate and drizzle with the sauce.