Heat the olive oil in a large skillet over medium-high heat. Add the lamb chunks and brown them on all sides, about 5 minutes total. Transfer the lamb to the slow cooker.
Add the diced onion and minced garlic to the same skillet and cook for 3 minutes until softened. Stir in the harissa paste and cook for 1 minute more.
Transfer the onion mixture to the slow cooker. Add the sweet potato cubes, diced tomatoes with their juice, beef broth, salt, and black pepper.
Stir everything together, cover, and cook on low for 7 to 8 hours until the lamb is fork-tender and the sweet potatoes are soft.
Taste and adjust salt if needed. Stir in the chopped cilantro just before serving.