In a bowl or measuring cup, combine the soy sauce, rice wine vinegar, Sriracha sauce, green scallions, garlic and ginger. Add the marinade over the tuna steaks in a shallow dish. Cover and let sit in the fridge for up to 2 hours, spooning the marinade over the tuna periodically.
Remove the tuna from the fridge and let sit in the marinade outside of the fridge for 20 to 30 minutes so the steaks can come up to room temperature. When the tuna has come to room temperature, remove it from the marinade. Gently scrape off some of the marinade when ready to sear the tuna. Sprinkle the top surface with the mixture of sesame seeds.
Bring your grill pan (or grill) to very high heat. When the pan is VERY hot but not quite smoking, add the tuna steaks, sesame seed side DOWN, and sear for about 4 - 5 minutes on the first side. Don't try to move them during this time. While searing, sprinkle the top (naked) side of the tuna with additional sesame seeds. Flip the steaks and sear on the 2nd side for about 3 minutes. The 2nd side will not take as long to cook as the meat will have begun to heat up by this point.
Remove the tuna from the pan, and let them rest for 4 to 5 minutes. Garnish with additional green onions if you'd like.
If you desire a sauce for the tuna, you could bring the marinade to a boil for 2 - 3 minutes in a small saucepan and serve on the side. For a thinner dipping sauce, boil "as is." For a thicker sauce, add a tiny bit of cornstarch (1/4 tsp ?) before heating, then bring to a boil. By boiling for a few minutes, you will kill any bacteria from the raw meat and render the sauce safe to eat. If this makes you too nervous, you could just double the marinade recipe and separate it prior to adding the meat.