Saute onion in oil; remove from heat. Stir in lemon juice, soy sauce, Worcestershire sauce, and mustard; pour over meat and vegetables. Cover and marinate overnight in refrigerator.
Remover meat and vegetables from marinade, reserving marinade. Alternate meat and vegetables on skewers. Grill 5 minutes on each side over medium coals or until desired degree of doneness, brushing frequently with marinade.
