Pat the lamb dry with paper towels. With a small paring knife, make a deep slit through the fat layer on the roast and insert a sliver of garlic and a rosemary leaf cluster. Repeat every 2-inches over the fat layer, using all of the garlic and rosemary. Sprinkle the roast with the cracked pepper and lavender. Cover and refrigerate overnight.
Remove the meat from the refrigerator and let sit at room temperature for 30 minutes before roasting. Meanwhile, position a rack in the center of the oven and heat the oven to 375°F.
Toss the potatoes with the olive oil in a 10x15-inch (or similar) roasting pan until well coated. Season with salt and pepper and arrange cut side down in a single layer.
Sprinkle the lamb all over with 1 Tbs. salt and set it on the potatoes. Roast until an instant-read thermometer inserted in the thickest part reaches 135°F for medium rare and the potatoes are tender when pierced with a fork, about 1-1/2 hours.
Transfer the roast to a serving platter or carving board, cover loosely with foil, and let rest for 20 to 30 minutes. Keep the potatoes warm in the turned-off oven.
Serve the roast whole or carved with the potatoes arranged around it.