Place the beef cubes in the slow cooker and sprinkle the flour, thyme, salt, and pepper over them. Toss to coat evenly.
Add the chopped onion and minced garlic to the slow cooker.
Pour in the stout beer, Dijon mustard, and beef broth. Stir everything together until the mustard is mostly dissolved.
Cover and cook on low for 7 hours.
Stir in the green beans, cover again, and cook for 1 more hour until the beans are tender but still bright.
Taste and adjust salt and pepper if needed before serving.