Trim any excess fat from the leg of lamb and cut into 1-inch cubes.
Heat the olive oil in a heavy-bottomed pot or tagine over medium-high heat.
Add the lamb cubes and sear until browned on all sides, then remove the meat and set aside.
Lower the heat to medium and add the chopped onion and minced garlic, sautéing for 3-4 minutes until translucent.
Stir in the cumin, coriander, cinnamon, turmeric, salt, and pepper, cooking for 1 minute until highly aromatic.
Return the lamb to the pot and add the vegetable broth, sliced carrots, and chickpeas.
Bring the liquid to a gentle simmer, cover with a tight-fitting lid, and cook for 25 minutes.
Add the diced zucchini to the pot, cover again, and continue simmering for another 10-15 minutes until the lamb is tender and vegetables are cooked through.
While the tagine finishes, place the dry couscous in a small heat-proof bowl and pour over 3 tablespoons of boiling water; cover and let sit for 5 minutes.
Fluff the couscous with a fork and serve the lamb tagine over the top, garnished with freshly chopped cilantro.