Preheat the oven to 160°C fan / 180°C conventional / 350°F.
Combine the flour, salt and black pepper in a bowl. Add the beef and toss until well coated.
Heat the fat in a large frying pan over medium heat. Brown the beef in batches, leaving it undisturbed for a couple of minutes before turning so it develops a good colour. Transfer each batch to a casserole dish, adding a little more fat to the pan as needed.
Add the onions to the same pan and cook for a few minutes, stirring and scraping up the browned bits from the bottom over a medium-low heat.
Pour in the beef stock and bring to the boil, stirring well to deglaze the pan. Add the bay leaf, then pour everything over the beef in the casserole dish.
Cover and cook for 1½ hours, or until the beef is tender.
Remove the bay leaf and gently stir the filling. Stir the cornflour slurry, then gradually add it to the filling, stirring gently until the gravy is thick enough for a pie. You may not need all of the slurry, or you may need a little more, depending on how much the gravy has reduced. If necessary, mix a little extra cornflour with an equal amount of cold water.
Taste and adjust the seasoning if needed. Add a few drops of Parisienne Essence if you would like a darker gravy.
Transfer the filling to a bowl and leave to cool to room temperature before assembling the pie. Alternatively, refrigerate the filling if making it ahead.