
In a small bowl, combine soy sauce, sugar, red pepper flakes, and broth. Sprinkle fish with salt and black pepper. Heat cooking oil in a large frying pan over moderately high heat. Add tuna and cook until brown, about 3 minutes. Turn and cook fish until done to your taste, 3 to 4 minutes longer for medium rare. Transfer tuna to a cutting board to rest while you make the sauce.
Reduce heat to moderately low and add sesame oil to pan. Stir in scallions and garlic and cook, stirring, until fragrant; about 1 minute. Add soy sauce mixture to pan and simmer until reduced to approximately ⅓ cup, about 2 minutes. Cut tuna into slices and serve with sauce.