Heat pan on stove to high/medium heat. Season steak with salt and pepper, cook in batches in pan until browned nicely, place into slow cooker.
Add stock, vinegar, sugar, honey, mustard, worcestershire sauce, tomato paste, cayenne pepper, chilli flakes and garlic to slow cooker. Making sure the beef is covered, add extra water if needed. Cook for 8 hours on low or 4-5 on high until meat falls apart when pinched.
When meat is cook, remove from stock and set aside. Pour liquid into a saucepan and reduce on simmer until its a thick consistency. Season to taste.
Combine spinach, cabbage, carrots and parlsey in a large bowl. Whisk mayo, vinegar, olive oil, salt and pepper together and toss over salad.
Serve beef on top of salad and dress with a few sprinkles of walnuts. Enjoy!