Place the potatoes in a 1-quart saucepan, and add water to cover by 1 inch. Set the pan over high heat, and bring to a boil. Reduce the heat; cook the potatoes at a gentle boil until barely fork-tender, about 10 minutes. Drain, and allow the potatoes to cool. Cut the potatoes in half; set aside.
Remove the stem and gills from the portobello with a sharp knife, and cut the cap into 8 wedges. Place the wedges in a medium bowl.
Cut the steak into 8 equal pieces, and season them with 1/2 teaspoon of salt and 1/2 teaspoon 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 the bowl with plastic wrap, and refrigerate for 30 minutes to 2 hours.
Heat a grill to medium-high, and lightly oil the grate.
Remove the bowl from the refrigerator. Skewer a potato, followed by a piece of steak and a mushroom wedge. Add another potato, piece of steak and mushroom wedge. Repeat with remaining skewers. Reserve remaining marinade for basting the skewers as they cook.
Grill the kebabs, basting them as they cook, for about 3 minutes. 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. Season the kebabs with the remaining salt, and serve immediately.