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.
