Season the shanks generously on all sides with the salt and black pepper, and lightly with the turmeric. Place the flour on a large plate. Dredge the shanks in the flour and shake to remove any excess.
Heat 3 tablespoons oil in a Dutch oven or large, heavy saute pan over high heat. Add the shanks and sear until well-browned on all sides, 8 to 10 minutes. Remove the shanks. Add the remaining teaspoon of oil, then the cinnamon, cumin, ginger, and cayenne, and cook, stirring, until fragrant, about 30 seconds. Add the onions and carrots and cook, stirring, until the onions are starting to color, 5 to 6 minutes. Add the garlic and cook, stirring, for 30 minutes.
Add the potatoes and cook for 1 minute. Add the wine and stir to deglaze the pan, cook until reduced slightly, 3 to 5 minutes. Add the tomatoes and cook, stirring, for 2 minutes. Return the shanks to the pot, add the stock, cilantro, parsley, orange juice, and orange peel, and bring to a boil. Reduce the heat to medium-low, cover with the lid slightly ajar, and simmer until the shanks are tender and the meat starts to fall from the bones, 1 hour and 45 minutes to 2 hours, skimming occasionally to remove any scum that forms on the surface.
Remove from the heat. Divide the couscous among 4 plates and top with the lamb shanks. Drizzle with the sauce and serve immediately.