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Place the potatoes in a large pot; add water to cover by 1 inch. Bring to a boil and cook until barely fork-tender, about 10 minutes. Drain, and allow to cool. Cut the potatoes in half; set aside.
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While the potatoes are cooking, remove the stems from the mushrooms and place the caps in a large bowl.
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Cut the steak into smallish pieces, and season them with salt and pepper. Place the meat in the bowl with the mushrooms. Add olive oil, balsamic vinegar, rosemary and garlic, and stir to coat mushrooms and steak well. Cover and refrigerate for 30 minutes to 2 hours.
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Heat a grill to medium-high, and lightly oil the grate.
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Put the meat, potatoes, and mushrooms on the skewers in whatever order you prefer. Reserve the remaining marinade for basting while grilling.
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Grill the kebabs for about 3 minutes, basting them once. Turn and cook for another 3 minutes, basting, before turning them over to cook on the first side again. Continue to cook on the first side for 2 to 3 minutes before turning them over and cooking a final 2 to 3 minutes.