Preheat an oven to 150°c/130°c fan (302°f/266°f).
Start making the braising liquid when the ribs have almost finished smoking. To do this, set a large heatproof saucepan over a medium heat, add in the vegetable oil then leave to heat up for a minute or so.The braising liquid will take around 30 minutes to make, including prepping the veg. Next, add the onions, carrots & celery into the pan, along with a pinch of salt then cook, stirring frequently until the veg has started to soften & has browned lightly. This will take around 10 minutes.
Whilst the veg is frying, use a piece of butcher's twine to tie the rosemary & thyme sprigs together then set to one side. This will make it easier to take the herbs out of the sauce later on.
Once the veg is cooked, add in the grated garlic then fry for another 30 seconds. Then stir in the tomato paste, cook for another 30 seconds then add in the Guinness, stirring to deglaze the pan.
Next, add in the beef stock, the bundle of herbs & the bay leaves then bring the stock to a simmer. Take the pan off the heat, add in the beef ribs then cover the pan with foil (or a heatproof lid).
Place the pan into the preheated oven then cook the ribs until the meat is tender & the bones slide out easily. This will roughly take 2-3 hours.You'll be able to tell when the ribs are done, when the meat is falling off the bones! Once the ribs are done, carefully lift them out of the stock then place them onto a tray & set to one side. Take the bundle of herbs & bay leaves out of the sauce & discard.
Set the pan of stock over a medium heat & bring to the boil. Then in the meantime, add the cornflour & cold water into a small mixing bowl and stir to make a smooth paste.
Once the stock is boiling, stir in the Worcestershire sauce followed by the cornflour paste then cook out for 2-3 minutes, until thick. Take the pan off the heat.
Next, use your hands to pull the short rib meat, keeping it fairly chunky then add into the sauce. Make sure to discard any bones & sinew.A pair of food safe gloves come in handy here! You could use the bones to make a beef stock. To finish, stir the meat into the sauce then season to taste with salt & pepper. Then transfer the pie filling to a shallow dish, leave to cool then chill in the fridge overnight.Chilling the pie filling overnight will improve the flavour & also ensures that it will be completely cold when we assemble the pie. The short rib rub is fairly peppery so you probably won't need to add much extra to the sauce.