Combine soy sauce; mirin, sake, or sherry; sugar; mustard; and garlic.
Cut partially frozen meat into thin strips.
Thaw meat completely.
Add to marinade and let stand 15 minutes at room temperature.
Thread meat strips accordion-style on small skewers.
Grill over hot coals to desired doneness, 5 to 7 minutes.
Turn and baste with marinade frequently.