Place the lamb pieces in the slow cooker. Add the sliced onion, carrots, minced garlic, chickpeas, and dried apricots.
Sprinkle the cumin, coriander, cinnamon, paprika, ginger, salt, and pepper over the ingredients. Pour in the diced tomatoes and beef broth. Stir gently to combine everything.
Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours, until the lamb is very tender.
About 10 minutes before serving, bring the water and olive oil to a boil in a small saucepan. Stir in the couscous, cover, and remove from heat. Let it sit for 5 minutes, then fluff with a fork.
Spoon the couscous into bowls. Top with the lamb tagine and its sauce. Scatter chopped cilantro over each serving and add lemon wedges on the side.