Mix the oil, soy sauce, vinegar, garlic salt, parsley and thyme to make a marinade. Save half of the marinade for later, and then cube the beef and add to the other half of the marinade.
If you are using bamboo skewers now would be a good time to start soaking them, since they need to soak for at least 30 minutes. Let the beef marinade in the refrigerator for at least 1 hour.
Prepare the onions, mushrooms and peppers by cutting them to size. Cook the potatoes, either in the microwave or by boiling them.
Thread the beef and vegetables alternately on the skewers in whatever pattern you like. Then brush the skewers with the reserved marinade.
Grill on a pre-heated grill for about 12 minutes, turning twice. Let rest 5 minutes before serving.