Heat the oil in a frying pan over medium heat. Fry the onion for 3-4 minutes, until softened and starting to colour. Stir in the garlic paste for the last 30 seconds.
Transfer the onion and garlic to the slow cooker along with the carrots.
Season the beef heavily on all sides with salt and black pepper.
Turn the heat to high and sear the beef in the same pan, about 30-45 seconds per side, until browned all over. Add it to the slow cooker.
Cut of beef. The original card just said "beef." A piece that's seared whole and then shredded needs a braising cut — chuck, braising steak, or brisket. Don't use a lean roasting joint like topside or silverside; it will go stringy rather than tender.
No gravy granules? Whisk 1½ tbsp / 12 g cornstarch with 2 tbsp / 30 ml cold water into a slurry and stir it into the hot liquid, or use a roux of 1 tbsp / 14 g butter and 1 tbsp / 8 g flour. Gravy granules are a UK pantry staple and already contain salt and colouring, so the cornstarch version may need extra seasoning.
Salt. Stock plus a stock cube plus Worcestershire plus gravy granules is a lot of salt stacked up. Use low-sodium stock if you're using all four, and season the beef itself confidently since that's the only salt that gets inside the meat.
Liquid level. Slow cookers don't evaporate, so the 400 ml goes in thin and stays thin — that's why it gets thickened at the end rather than reduced. Don't add extra liquid at the start even if it looks like the beef isn't covered.
Searing is worth the pan. 30-45 seconds a side on high heat is all it takes, and it's the difference between beef that tastes braised and beef that tastes boiled.