Prepare: Wash the beef, slice it into cubes, and refrigerate until needed. Peel and cube the potatoes. Dice the onions and garlic. Heat cooking oil in a pan over medium-low heat, then sauté the onions and garlic until golden brown, about 5-8 minutes.
Cook: In a cup, mix hot water with the beef cube, Worcestershire sauce, soy sauce, and brown sugar until dissolved. Add the beef cubes and half a cup of the mixture to the pot, then incorporate the potatoes and the remaining liquid. Simmer for 20 minutes (longer for tougher beef cuts).
Bake: If using large puff pastry, cut it into squares. Add two tablespoons of filling to each square, fold into rectangles, and seal edges with a fork. Whisk an egg and brush over the pastries for a golden finish. Optionally, sprinkle with sesame or poppy seeds. Bake at 180°C/375°F for 40 minutes. Let cool for 5 minutes before serving.