Heat some oil in a large pan. I like to use a cast-iron pan with a tight-fitting lid.
Season the beef generously with sea salt and freshly ground pepper.
Brown the beef in small batches, and use a slotted spoon to remove the browned meat to a large bowl. Try not to over-crowd the pan, otherwise the meat will steam instead of brown; it is the caramelisation on the meat which gives a lot of flavour to the stew.
Add the onions and garlic, and cook for 1-2 minutes.
Add the Guinness and let everything bubble away.
Use a wooden spoon to scrape off any caramelised bits on the pan.
Add the beef stock, thyme and bay leaves.
Return the beef to the pan, together with any meat juices which have collected in the bowl.
Turn the heat down to low, and cook gently on a low simmer for about 2 hours, or until the beef is tender. See Kitchen Notes below for instructions using a pressure cooker.
Once the beef is tender, add the mushrooms.
Simmer gently for a few minutes until the mushrooms have softened slightly.
During this time, try to remove the thyme and bay leaves.
Turn up the heat to medium-high, and slowly add the cornflour slurry. You may not need all of it - just add enough until you have a fairly thick consistency.
Taste the sauce for seasoning.
Stir through the parsley.
Set the mixture aside to cool completely.