Heat the olive oil in a large heavy-bottomed pot over medium heat.
Add the cubed lamb leg and sear until browned on all sides, then remove and set aside.
Add the diced onion and sliced carrots to the pot, sautéing until the onions are translucent.
Stir in the cumin, coriander, cinnamon, salt, and pepper, cooking for one minute until the spices become fragrant.
Return the lamb to the pot along with the tomato puree, beef broth, and rinsed chickpeas.
Bring the mixture to a boil, then reduce heat to low, cover, and simmer for 45 minutes until the lamb is tender.
While the tagine simmers, prepare the couscous by following the package instructions until fluffy.
Serve the lamb tagine over the couscous and garnish with freshly chopped parsley.