Heat the olive oil in a large skillet over medium heat. Add the sliced onions and cook for 8 to 10 minutes, stirring often, until they soften and turn light brown.
Add the beef pieces to the same skillet and cook for 3 to 4 minutes until the outside is no longer pink.
Transfer the onions and beef to the slow cooker. Add the minced garlic, salt, and black pepper.
Pour in the beef broth and tuck the thyme sprigs into the liquid.
Cover and cook on low for 7 to 8 hours until the beef is tender.
Remove the thyme sprigs before serving.