Heat the olive oil in a large skillet over medium-high heat. Add the beef cubes and brown on all sides, then transfer them to the slow cooker.
Sprinkle the flour over the beef in the slow cooker and stir to coat.
Add the mushrooms, celery root, carrots, onion, and garlic to the slow cooker.
Pour in the stout beer and beef broth. Add the thyme along with salt and pepper.
Stir everything gently to combine, then cover and cook on low for 8 hours until the beef is tender and the vegetables are soft.