Heat the olive oil in a large tagine or heavy-bottomed Dutch oven over medium-high heat. Add the lamb chunks and brown them on all sides, about 5 to 7 minutes. Remove the lamb and set aside.
In the same pot, add the chopped onion and sauté until softened and translucent, about 5 minutes. Stir in the minced garlic, cumin, and cinnamon, cooking for another minute until fragrant.
Return the browned lamb to the pot. Stir in the harissa paste, beef broth, and diced tomatoes, scraping up any browned bits from the bottom. Bring the mixture to a gentle simmer.
Cover the tagine or pot tightly, reduce the heat to low, and let it slow-cook for 1 hour, stirring occasionally.
Add the cubed potatoes and sliced carrots to the stew. Cover again and continue to simmer for another 40 to 45 minutes, or until both the meat and vegetables are completely tender.
Taste and adjust the seasoning if necessary. Garnish with freshly chopped parsley before serving hot with couscous or crusty bread.