Cut the leg of lamb into even cubes, about 2x2 cm, for uniform cooking.
In a small bowl, mix the allspice, cayenne pepper, cumin, thyme, rosemary, garlic powder, and a small pinch of salt.
Rub the spice mixture evenly over the lamb cubes and let them marinate for 30 minutes to soak up the flavors.
Thread the marinated lamb onto skewers, leaving small gaps between pieces for even grilling.
Brush the skewers lightly with vegetable oil to prevent sticking.
Preheat your barbecue for around 15 minutes. Place the skewers on the grill over medium heat.
Grill for 10-15 minutes, turning occasionally, until the lamb is golden-brown and slightly charred on all sides. Look for a juicy center, but cook longer if you prefer well-done.
While the lamb is grilling, prepare the mint sauce. In a bowl, mix the finely chopped mint, garlic, crème fraîche, olive oil, lime juice, and zest. Add white pepper and a pinch of salt to taste.
Once the lamb skewers are done, remove them from the grill and arrange them on a serving platter. Add lime wedges on the side for squeezing over the top.
Serve the skewers hot, with the mint sauce on the side for dipping or drizzling.