Heat a large skillet over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon as it cooks.
Add the diced onion and minced garlic to the skillet. Stir and cook for 3 minutes until the onion softens.
Transfer the beef mixture to the slow cooker. Add the cubed sweet potatoes, beef broth, diced tomatoes, dried thyme, salt, and black pepper.
Stir everything together, cover, and cook on low for 6 to 7 hours until the sweet potatoes are tender.
Stir in the chopped kale during the last 20 minutes of cooking. Cover again and let the kale wilt before serving.