1. Preheat oven to 450F. Peel and quarter potatoes.
2. Fill a pot with enough water to cover potatoes and bring to a boil.
3. Immerse potatoes and parboil for 5-10 minutes. Potatoes should be slightly soft on the outside and hard in the middle.
4. Meanwhile, remove rosemary leaves and discard stems. Finely chop leaves and set aside.
5. Drain water from pot and discard, or reserve for another use. Add flour, salt, and pepper to potatoes.
6. Place a lid on the pot and shake vigorously so that the potatoes look a little mushy on the outside. Set aside.
7. Place butter on a rimmed baking sheet and heat in oven for 3 minutes, or until fat is sizzling.
8. Carefully remove pan from oven, add potatoes, then tilt pan until fat collects in one corner. Gently spoon fat on each individual potato. Roast for 30 minutes.
9. With a heatproof spatula, scrape potatoes off the sheet (to keep the crispy skin on the bottom) and turn over.
10. Roast for another 30 minutes or until golden brown.
11. When ready to serve, add the rosemary and stir to combine. Place in oven for another 2 minutes, and serve.