Preheat oven to 180 degrees C. Brown the stewing steak in a frying pan, then transfer to a casserole dish.
Make a pint of beef stock using the stock cube, then stir in the gravy granules, using 4-5 tsp depending on your preference. Pour stock mix over the browned stewing steak and bake in the preheated oven for about an hour.
Meanwhile, roll out the puff pastry and cut four pieces into your desired size. Brush the milk over the pastry for a glaze. Put the pastry slices on a baking tray and bake near the top of the oven halfway during the cooking time of the steak.
When ready, spoon the steak onto the plates and place a puff pastry slice on top of each portion. Serve with mashed potatoes and veggies.