Heat the olive oil in a large skillet over medium-high heat. Add the beef stew meat and brown on all sides, about 5 minutes total. Transfer the meat to the slow cooker.
Add the diced onion and minced garlic to the same skillet. Cook for 2 minutes until the onion softens, then stir in the flour and cook for 1 minute more.
Pour in 1 cup of the beef broth and scrape the bottom of the skillet to loosen any browned bits. Transfer this mixture to the slow cooker.
Add the sliced mushrooms, thyme leaves, potato cubes, carrot slices, remaining beef broth, salt, and black pepper to the slow cooker. Stir to combine.
Cover and cook on low for 8 hours until the beef is tender and the vegetables are soft. Stir once halfway through if possible.