How to Make It
Preheat the oven to 375°. In a large skillet, heat 1/4 cup of the oil until shimmering. Add the lamb and cook over moderately high heat, turning occasionally, until browned, 8 minutes. Transfer to a medium baking dish. Season with pepper; let stand for 10 minutes.
Meanwhile, in a medium bowl, combine the 5 cups of kosher salt with the thyme, rosemary and lavender. Add the egg whites and stir until the salt is evenly moistened.
Pack the salt all over the lamb in the baking dish, leaving no cracks. Roast in the center of the oven for about 50 minutes, until an instant-read thermometer inserted (through the crust) in the roast registers 120° for medium-rare. Let the lamb rest for 10 minutes, then crack the crust and remove it.
Meanwhile, in a large skillet, heat the remaining 1/2 cup of oil. Add the potatoes, season lightly with salt and cook over moderate heat, turning occasionally, until thoroughly softened but not browned, about 8 minutes. Using a slotted spoon, transfer the potatoes to a platter; season with pepper.
Remove the strings and carve the meat into thin slices. Serve with the potatoes.