Before you sear the tuna, be sure to get the pan screaming hot--this gives you a deep crust without overcooking the interior. Look for yellowfin tuna caught using the troll or pole-and-line technique.
1. Combine first 9 ingredients in a medium bowl.
2. Combine soy sauce and brown sugar in a small bowl, stirring until brown sugar dissolves. Reserve 1 tablespoon soy sauce mixture. Heat a grill pan over high heat. Coat pan evenly with cooking spray. Add steaks to pan; cook 2 minutes on each side or until desired degree of doneness, basting steaks frequently with soy mixture. Remove steaks from grill pan; drizzle with reserved 1 tablespoon soy mixture. Serve fish with salsa.