Instructions

Gather the ingredients.
Grilled tuna steak ingredients
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.
Combine the soy sauce, chopped scallions, lemon juice, sesame oil, and fresh ginger in zip top bag
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.
Add tuna steaks and marinate in zip top bag
Preheat a countertop contact grill to "sear" or the highest temperature setting.
Place the sesame seeds on a plate or a shallow dish. Remove the tuna steaks from 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.
Coat marinated tuna steaks with sesame seeds
Spray the grill plates lightly with nonstick 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 (longer if you prefer your tuna cooked through). Remove from grill and keep warm.
Place tuna steaks on grill and cook
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.
Flip tuna steaks on grill
While the tuna cooks, pour the marinade into a small saucepan and bring to a boil. Add the cornstarch, stirring with a whisk. Cook for about 3 to 4 minutes, until the sauce thickens.
Cook marinade for tuna steaks to make a sauce
To serve, slice the tuna steaks on the diagonal into thin slices. Fan the slices on a plate and drizzle with the sauce.
Finished tuna steaks
Enjoy!