Heat up the oil in a large pan or a wok, add half of the beef chunks and season with salt and pepper.
Sear on both sides, then transfer the beef to the slow cooker, and repeat with the remaning beef - this is avoid overcrowding the pan and steawing the beef rather than searing it.
Once all the beef has been transferred to the slow cooker, add the peeled and chopped garlic, chopped spring onions and ginger.
In a small bowl, combine the soy sauces, sesame oil, rice vinegar and sugar.
Mix well, then add the sauce to the slow cooker together with the beef stock.
Place a lid on the slow cooker and leave it cook on high for 3 ½ hours or on low for 7 ½ hours.
Mix the cornflour with 2 tablespoons of water and whisk it well to dissolve the cornflour.
Add the slurry to the slow cooker and give it a good stir.
Add the broccoli cut into florets, and place the lid back on.
Leave it to cook for a further 30 minutes.
Garnish with seasame seeds.