Rinse the lamb well under cold water and pat it very dry with paper towel. Using a small knife, make 16 1-inch deep incisions across the top side (skin side) of the lamb leg. Stuff each incision with ½ of a garlic clove and a small amount of rosemary. Drizzle both sides of the lamb with olive oil and massage evenly to coat. Generously season all over with kosher salt and black pepper. For best results, set the lamb onto a sheet pan lined with a wire rack and refrigerate uncovered for 6 hours to overnight.

Preheat your oven or Traeger Grill to 350F.

Transfer the lamb to a large roasting tray. In a separate bowl, combine the chicken broth, white wine (if using), lemon juice, mustard and oregano and whisk to combine. Pour the mixture around the lamb. Cover the lamb with a sheet of wet parchment paper and seal the tray tightly with aluminum foil. Transfer the roasting pan to the oven or Traeger and cook for around 3.5 hours or until the lamb is very tender and can be easily shredded with a fork. If the lamb is still firm after 3.5 hours, reseal the foil and continue cooking, checking on it every 30 minutes.

Remove the aluminum foil and parchment paper and baste the lamb with the broth mixture. Add the potatoes to the roasting pan and toss them to coat in the juices. Return the uncovered roasting pan to the oven or Traeger Grill, raise the heat to 425F and cook until the lamb is deeply browned and the potatoes are fork-tender, around 35 minutes. If the broth evaporates too much, add more as needed (the rate of evaporation will depend on the size of your roasting tray). If the lamb starts to burn, cover it with the parchment paper and foil (the rate of browning will depend on the strength of your oven or grill).

Transfer the roast lamb and potatoes to a platter and pour the pan gravy into a separate jar. Let the lamb rest 10 minutes before serving and let the gravy settle for 5 minutes before skimming off excess fat with a ladle.