In a small bowl, whisk together the soy sauce, rice vinegar, sesame oil, ginger, garlic, brown sugar, lime juice, and green onions. Pour half of the mixture over the tuna and cover with a lid. Refrigerate for at least 1 hour. Once marinated, coat the tuna with sesame seeds. Discard the marinade that the tuna was marinating in. In a nonstick skillet, heat the oil over medium-high heat. For rare tuna, cook for 2 minutes per side. For well-done tuna, after cooking it for 1 to 2 minutes per side,
