Prepare the Filling: Brown the minced beef and bacon in a large pan. Add onions, parsnips, carrots, leeks, salt, pepper, and garlic, cooking until the onions are soft and golden.
Thicken the Sauce: Sprinkle flour over the mixture and cook for 3-5 minutes. Stir in tomato paste and cook for another minute. Add Worcestershire sauce, marmite, and water, then simmer for 5-10 minutes.
Finalize Filling: Add parsley to the meat sauce, adjust seasoning with salt and pepper, and remove from heat. Let it cool before using.
Assemble the Pie: Preheat oven to 190-200°C. Line a pie dish with puff pastry, add the cooled filling, and cover with another layer of pastry. Seal edges and brush the top with beaten egg.
Bake: Make small cuts in the top layer to allow steam to escape. Bake for 25-30 minutes or until golden and crisp.
Serve: Let the pie rest for a few minutes before serving.