Season:
In a small bowl, combine the paprika, ginger, turmeric, cayenne, mint, coriander, cumin, 2 tsp. salt, and 1 tsp. pepper. Rub the shanks with 2 Tbs. of the spice mixture (save the rest for the braising liquid). Cover and refrigerate for at least 2 hours but preferably overnight.
Sear:
Prepare a gas grill for direct grilling over medium heat. Grill the lamb shanks until golden brown, about 4 minutes per side. Let cool briefly and then tie each shank with 3 or 4 loops of butcher’s twine.
Braise:
Prepare the grill for indirect grilling. In an 8-quart heavy-duty pot, combine the onion, tomatoes, cilantro stems, lemon juice, garlic, and the remaining spice mixture. Stir in 4 cups water and 1 tsp. salt. Nestle the lamb shanks into the pot in a single layer. Set the pot on the grill over the cool zone. Cover the pot and close the grill lid. After 1 hour, add the carrots, turnips, and chickpeas. Cover the pot and the grill, and braise until the lamb shanks and vegetables are fork-tender, an additional 1/2 to 1 hour.
Transfer the shanks to a tray. With a slotted spoon, transfer the vegetables and chickpeas to a bowl and keep warm.
Pour the braising liquid into a heat-proof vessel, such as a Pyrex measuring cup, and let sit until the fat rises to the top. Skim off and discard the fat. Taste the liquid; if it’s too watery, return to the pot and boil over high heat until flavorful. Season to taste with salt, pepper, and lemon juice. Keep warm.
Glaze:
Prepare the grill for direct grilling over medium heat. In a small saucepan, combine 3 Tbs. of the braising liquid with the honey, lemon juice, mint, and harissa (if using). Boil over medium heat until the mixture has the consistency of a light syrup, 1 to 2 minutes.
Brush the mixture on the shanks and then grill them, turning and basting until the glaze has bubbled and has a few black spots, about 5 minutes total.
To serve, remove the strings from the shanks. Mound the couscous in a large shallow serving bowl. Spoon the vegetables and chickpeas over the couscous. Set the shanks on top, and sprinkle with the cilantro. Season the remaining braising liquid to taste with harissa (if using) and serve on the side.
