Cut the steak into 16 equal pieces if using a thin cut and 8 equal pieces if using a thick cut. Season with ½ teaspoon salt and ½ teaspoon pepper. Place the steak and mushrooms in a large bowl. Add the olive oil, vinegar, rosemary, and garlic, and stir until the steak and mushrooms are well coated. Cover with plastic wrap and refrigerate at least 30 minutes to 2 hours.
Place the potatoes in a large saucepan and add enough water to cover by 1 inch. Bring the water to a boil and cook potatoes until just barely fork-tender, about 5-7 minutes. Drain and set aside.
Add a piece of steak, a potato, another piece of steak, and a mushroom to a skewer. Then add another piece of steak, a potato, another piece of steak, and a mushroom again. Repeat with remaining 3 skewers and ingredients.
Heat a grill to medium-high and lightly oil the grate. Add the kabobs and grill about 5-6 minutes on both sides, flipping every 3 minutes. Sprinkle kabobs with remaining 1 teaspoon of salt and serve immediately.
Notes
- If using wood skewers, soak them in warm water for at least 30 minutes before using so they don't burn on the grill. - Clean the mushrooms before they go in the marinade by wiping any dirt away with a damp paper towel.
