1. Two hours before cooking the lamb, puncture the meat on all sides with a paring knife, and stud with the quartered garlic cloves. Reserve, chilled.
2. Place the potatoes in a large saucepan, cover with cold salted water, and simmer until tender. Drain and, when cool enough to handle, peel the skins and set the potatoes aside.
3. Fill a large bowl with cold water and squeeze in the juice from the lemons. Clean the artichokes one at a time by peeling away the outer leaves and trimming the stems to about 1 1/2in (3cm) in length. Using a sharp paring knife, remove the skin from the stems and any tough leaf remnants from the base. With a spoon, scrape out and discard the chokes. Cut the artichokes in half. Immediately submerge in the lemon water after preparing each artichoke.
4. Preheat the oven to 300°F (149°C). Rest the lamb at room temperature for 30mins and pat dry. Season liberally on all sides with salt, white pepper, and piment d’Espelette. Heat the olive oil in a roasting pan over high heat.
5. Sear the leg on all sides until browned, about 12mins total. Reduce the heat to medium and add the butter, garlic cloves with skin, sage, and bay leaf. Roast, turning and basting the leg often with the butter, especially in areas that don’t touch the pan directly, about 5mins total.
6. Transfer to the oven and roast for 35mins. Remove from the oven and transfer the lamb to a platter.
7. Strain the artichokes, pat dry, and add to the roasting pan with the onions, and set over medium heat. Cook, turning the vegetables on all sides to brown, about 5mins. Add the potatoes, toss, and season with salt and pepper. Continue cooking, stirring often, for five more minutes.
8. Nestle the lamb into the vegetables and return the roasting pan to the oven. Roast until the temperature at the meatiest part of the leg reaches 125°F (52°C), about 45mins.
9. Transfer the leg to a platter, cover with aluminum foil, and rest in a warm place for at least 20mins before slicing. If the vegetables aren’t tender, return them to the oven to finish cooking.
10. Scoop the vegetables into a serving dish; cover and keep warm. Set the roasting pan over medium heat. Add the wine and reduce until almost dry, using a wooden spoon to scrape the bottom of the pan. Add the chicken stock and reduce by half. Season with salt and pepper and pour into a serving bowl; use for saucing the meat.
11. Toss the watercress with the vinaigrette and serve alongside the lamb with the vegetables and pan sauce.