For the sausage: Place the lamb, harissa, ginger, coriander, salt, cinnamon, pepper, sumac, turmeric and garlic in the bowl of a stand mixer. Mix on medium speed for 2 minutes, then turn out onto a sheet of plastic wrap and form into a small, dense log. Wrap well and refrigerate for at least 24 or up to 72 hours.
For the stuffing: Preheat the oven to 400 degrees F. Grease a large casserole dish (such as a 9-by-13-inch or equivalent oval) with olive oil; set aside. Put the bread cubes in a very large bowl.
Place a large saute pan over high heat. Break the sausage meat into marble-sized pieces and brown on all sides in batches; remove and add to the bowl, letting the bread soak up any grease.
Once all the sausage is browned, reduce heat to medium-high (do not drain the grease from the pan). Add the sweet potatoes and cook, stirring occasionally, until they begin to brown, 4 minutes. Add the onions and peppers and continue to cook, stirring occasionally, another 4 minutes, then add garlic and walnuts, if using, and cook until fragrant, about 3 minutes. Pour the vegetables on top of the bread and sausage.
Whisk the eggs and turkey stock together in a large bowl, then add the turmeric, coriander, paprika, salt, cayenne, if using, cinnamon, cumin, ginger, black pepper and 2 tablespoons of harissa. Pour half evenly over the stuffing mixture and toss well; when all the liquid has been absorbed, add the chopped parsley and remaining stock and continue tossing until well combined. Taste for seasoning, adding more harissa and spices as you'd like.
Pour the stuffing into the prepared casserole dish, spreading out evenly, then cover with aluminum foil and bake for 40 minutes. Remove the foil and bake until brown on top, an additional 10 minutes. Allow to rest for 10 minutes, then serve warm.