Cut onions in 1 1/2-inch-long pieces.
Mix soy sauce, oil, lemon juice, sugar, 2 tablespoons water, and % teaspoon pepper.
Marinate beef and onion in soy mixture 30 minutes; drain.
On bamboo skewers alternately thread beef and onion pieces.
Grill over hot coals till meat is desired doneness (allow 12 to 15 minutes for medium).
Turn often; brush occasionally with reserved marinade.