Combine first 5 ingredients in a ziptop bag. Place steak in bag and refrigerate for 4 to 6 hours.
Preheat oven to 400º. Combine miniature potatoes and 1 tablespoon olive oil in a medium bowl. Use your hands to massage the oil all over the potatoes. Season with salt and pepper, place on a baking sheet and bake for 25 minutes. Remove from oven, stir potatoes, drizzle with 1 more tablespoon of oil, and sprinkle with rosemary and Parmesan Cheese. Place back in oven for 10 more minutes.
Fire up a gas grill or charcoal grill. When hot, remove shirt steak from marinade and place on grill. Grill 3-4 minutes per side. (Skirt steak is best served medium-rare.) Let rest before slicing against the grain.