Add the diced onion, minced garlic, sweet potato cubes, and chickpeas to the slow cooker.
Pour in the diced tomatoes with their juices and the vegetable broth.
Sprinkle the cumin, cinnamon, ginger, smoked paprika, salt, and black pepper over the top.
Stir everything together until the spices are evenly distributed.
Cover and cook on low for 6 to 7 hours, until the sweet potatoes are soft.
Stir the finished tagine, then serve in bowls topped with chopped cilantro and a squeeze of lemon.