Coat grill or skillet with cooking spray.
Season tuna fillets with salt and pepper. Place fillets on hot grill or skillet and cook 3 minutes on each side until medium rare or the desired degree of doneness. Remove from heat.
Combine broth, vinegar, sugar, soy sauce and cornstarch in a small saucepan. Bring to a boil and cook for 1 minute, stirring. Remove from heat and pour over tuna fillets.
Garnish each fillet with green onion.
How to fillet, skin and debone fish