In a large zip-top bag, combine soy sauce, chopped scallions, lemon juice, sesame oil and fresh ginger. Swish around in bag to combine. Add tuna steaks, turning within bag to coat, and marinate in refrigerator for about 20 minutes.

Preheat contact grill to "Sear" or highest temperature setting. Place sesame seeds on a plate. Remove tuna steaks from marinade bag, brushing scallions off the steaks and reserving marinade. One at a time, coat the steaks in sesame seeds on all sizeds, pressing the seeds into the steak so they'll stick.

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. Grill for about 3 minutes (longer if you prefer your tuna cooked through). Remove from grill and keep warm.

Meanwhile, pour marinade into a small saucepot and bring to a boil. Add cornstarch, stirring with a whisk. Cook for about 3-4 minutes, until sauce thickens.

Drizzle tuna steaks with sauce before serving.