Add the onion, garlic, ginger, soy sauce, honey, brown sugar, rice vinegar, sesame oil and beef stock to the slow cooker and stir to combine. Place the beef chuck on top, pop the lid on and cook on LOW for 8 hours or HIGH for 4–5 hours.
About 15 minutes before serving, use two forks to break the beef apart. Mix the cornflour and water to make a slurry, then stir it through along with the broccoli and capsicum. Replace the lid and cook on HIGH until the sauce thickens and the vegetables are just tender.
Meanwhile, cook the udon noodles according to packet instructions. Add the noodles to the slow cooker and toss everything together until glossy and coated in the sauce. Finish with sesame seeds and spring onion, then serve.
