12 large unpeeled garlic cloves, divided
1 tablespoon chopped fresh rosemary leaves
Kosher salt
Freshly ground black pepper
2 tablespoons unsalted butter, melted
1 (6-pound) boneless leg of lamb, trimmed and tied
4 to 5 pounds small unpeeled potatoes (16 to 20 potatoes)
2 tablespoons good olive oil