1. Place the diced lamb in a bowl, toss with 2 tablespoons of olive oil, and set to the side.
2. In a large resealable bag, toss together the paprika, turmeric, cayenne, cinnamon, cloves, cardamom, salt, ginger, saffron, garlic powder, and coriander and mix well. Add the lamb to the bag, and toss to coat well. Refrigerate for at least eight hours, preferably overnight.
3. Heat 1 tablespoon of olive oil in a large, heavy-bottomed pot over a medium-high heat. Add 1/3 of the lamb, and brown well. Place on a plate and repeat with the remaining lamb.
4. Add onions and carrots to the pot and cook for around 5 minutes. Stir in the fresh garlic and ginger, and cook for another 5 minutes.
5. Return the lamb to the pot and stir in the lemon zest, chicken broth, tomato paste, and honey. Bring to the boil, reduce to a low heat, and simmer for 1.5 to 2 hours, stirring occasionally, until the meat is tender. If the consistency of the tagine is too thin, you can thicken it using the cornstarch and water during the last 5 minutes.
6. Serve with couscous.