Except for the tuna steaks and salt and pepper, mix all the ingredients together in a medium bowl until the mixture is well-blended.
Rinse the tuna steak with water, pat dry, and season with salt and pepper on both sides.
Add the tuna steak to the marinade, turning once or twice with tongs to coat all sides. Marinate the tuna for 30 minutes in the refrigerator.
Heat the grill to medium-high heat; oil the grill. Grill the tuna steak for 1 minute on each side then place on a baking sheet. Continue to cook in a preheated oven at 250°C for 3 to 5 minutes or until the tuna is tender.
Sprinkle the tuna with chopped scallions just before serving.