Preheat the broiler and place shanks under the hot broiler for about 5 minutes on each side to brown and render a bit of the fat. While this is happening, chop the large onion and sauté in the olive oil along with the cumin, coriander, turmeric, salt, fennel seeds, and black and cayenne peppers until onion is soft.
In a slow cooker (or a Dutch oven) place the lamb shanks, the spiced onions, chicken stock, tomatoes, garbanzos, garlic, ginger, lemon peel, cinnamon stick, mint, 2 TB of chopped cilantro and half of the diced red peppers. Reserve the remaining red peppers and cilantro to add later. Cook at low heat in the slow cooker for 4 hours. Skim off any excess fat and then add the tomato puree and the remaining red peppers. Cook for 1 additional hour. Stir in the remaining cilantro and the spinach, turn the slow cooker up to high and when the spinach has wilted, serve over couscous. Garnish with slivered almonds.