Heat a large skillet over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon as it cooks. Drain excess fat.
Add the chopped onion and minced garlic to the skillet. Cook for 2 minutes until the onion softens.
Transfer the beef mixture to the slow cooker. Add the diced potatoes, sliced carrots, sliced celery, diced tomatoes, and beef broth.
Stir in the dried thyme, salt, and black pepper. Cover and cook on low for 7 to 8 hours.
Stir the stew before serving to check the consistency of the vegetables.