Trim any excess fat from the lamb leg and cut into 1-inch cubes.
Heat the olive oil in a heavy-bottomed pot or tagine over medium heat.
Add the lamb cubes and sear until browned on all sides, then remove the meat and set it aside on a plate.
In the same pot, add the diced onion and sauté for 4-5 minutes until translucent.
Stir in the minced garlic and grated ginger, cooking for 1 minute until highly aromatic.
Add the cumin, coriander, cinnamon, turmeric, salt, and pepper, stirring constantly for 30 seconds to toast the spices.
Pour in the chicken bone broth and return the lamb and any accumulated juices to the pot.
Add the chopped Medjool date and stir to combine.
Bring the liquid to a gentle simmer, then cover the pot with a tight-fitting lid and reduce heat to low.
Simmer for 45 to 60 minutes, or until the lamb is very tender and the sauce has thickened.
Garnish with freshly chopped cilantro before serving warm.