Preheat the oven to 150 °C. In a bowl, place the flour and steak. Use your hands or a spoon to toss the beef in the flour.
Heat a cast iron or baking dish on the stove to a medium temperature. Heat a generous splash of the avocado oil, then fry the pieces of steak until golden brown on all sides. Remove from the dish and set aside.
Leave the remaining oil in the pan, and add the leek, celery and crushed garlic. Cook for 5-10 minutes or until softened slightly, then lower the heat and stir in the tomato paste and worcestershire sauce, followed by the beef stock.
Add the beef back into the dish, stir to combine, then cover with a lid and place in the oven to cook for 3-4 hours.
Remove the stew from the oven, then increase the oven temperature to 200 °C.
Spoon the beef into a pie dish, then sprinkle the blue cheese over the top. Lay your pastry sheet over the top of the pie, pinching or folding/tucking in the edges to seal.
Brush the pastry with beaten egg, then sprinkle with sesame seeds. Bake in the oven for a further 20-25 minutes towards the bottom of the oven, until golden brown and puffed.