Combined Ahi tuna steak with one fourth cup of soy sauce and lemon juice of one lemon into plastic bag. Marinate for at least one hour, turning often. Remove tuna steak from marinade and put in dry bowl. Add black sesame seeds to the first side and let rest for 15 minutes. Get large spoon and press black sesame seeds into tuna. Turn tuna over and add sesame seeds to the other side. Let rest for another fifteen minutes. Press black sesame seeds into tuna with large spoon. While preparing tuna steaks, heat cast iron skillet over high heat for at least 20 minutes. Add to the tuna steak to hot pan and sear each side for two minutes. With sharp knife cut tuna steak into quarter inch slices. Serve tuna slices with remaining marinade and top with saffron aioli.