Add the onion, garlic, tomato purée, Worcestershire sauce, caramelised onion chutney, beef stock, thyme, salt and black pepper to the slow cooker and stir together. Add the beef joint and turn it through the sauce. Optional: For extra colour and flavour, sear the beef in a hot frying pan for 2-3 minutes per side before adding it to the slow cooker. Cover and cook on LOW for 6–8 hours, or until the beef is tender enough to slice easily.