How to Make It
In a bowl, combine the oil, rosemary, garlic, lemon zest, lemon juice, crushed red pepper and 1 tablespoon kosher salt. Let the chimichurri stand at room temperature for at least 20 minutes.
Preheat the oven to 500°. Season the lamb with salt and pepper and brush on both sides with 2/3 cup of the chimichurri. Set the lamb, fat side up, on a large rimmed baking sheet and roast in the upper third of the oven for about 25 minutes, or until an instant-read thermometer inserted in the thickest part of the meat registers 130° for medium rare. Transfer the lamb to a cutting board, cover with foil and let rest for 10 minutes.
Meanwhile, divide the butter between 2 large skillets, preferably cast iron. Add one-quarter of the potatoes to each skillet and cook over low heat until browned on the bottom, about 15 minutes. Turn the slices with tongs and fry until browned on the other side, about 5 minutes longer. Transfer the potatoes to a rack set over a baking sheet, season with salt and keep warm on the back of the stove. Fry the remaining potatoes.
Thickly slice the lamb. Arrange the potatoes on a plate, top with the lamb and spoon the chimichurri over it all.