Slice potatoes wafer thin. Rub rosemary between fingers to break leaves; set aside. Preheat oven to 425 degrees.
Heat oil in cast iron skillet. Add potatoes; cook over medium high heat, gently tossing potatoes. When potatoes are slightly limp, sprinkle with rosemary, mustard and pepper. Shake well to mix. Press with spatula. Cook until brown and crisp on bottom.
Sprinkle with cheese; place skillet in oven. Bake for 2-3 minutes or until cheese starts to bubble and brown.