Trim any excess fat from the lamb leg and cut into 1-inch cubes.
Heat the olive oil in a tagine or heavy-bottomed pot over medium-high heat.
Season the lamb with sea salt and black pepper, then sear in the pot until browned on all sides.
Add the diced onion, carrots, and minced garlic to the pot, sautéing for 3-4 minutes until the vegetables begin to soften.
Stir in the ground cumin, ginger, and cinnamon, tossing the meat and vegetables to ensure they are evenly coated in spices.
Pour in the beef broth and bring the mixture to a gentle boil.
Reduce the heat to low, cover with a tight-fitting lid, and simmer for 35 minutes.
Add the diced zucchini to the pot, cover again, and continue simmering for another 10 minutes until the lamb is tender.
While the tagine finishes, prepare the couscous by placing it in a small bowl and pouring 0.25 cup of boiling water over it; cover and let sit for 5 minutes before fluffing with a fork.
Spoon the lamb tagine and its aromatic sauce over the fluffy couscous and serve immediately.