Put the meat in a large bowl and cover with the flour. Toss the meat around with two forks and ensure it is well coated.
Pour a little olive oil in a large pot and brown the meat in. Remove the meat from the pot and set aside.
Add the onions to the same pot used to brown the meat and allow to soften slowly and gently.
Place the meat back in the pot with the onion. Pour in the stout and water and follow with the stock cube, Worcestershire sauce, mustard, mixed herbs and sliced mushrooms.
Bring the ingredients to the boil and then reduce the heat to slow simmer.
Simmer the stew on the stove for 2 1/2 to 3 hours or until the meat is very tender and the sauce has reached a thick consistency. Turn off the heat.
Pre-heat the oven to 180°C/350°F/Gas Mark 4
Make the pastry by melting the butter, water and salt in a saucepan, and then bring it to the boil. Place the flour in a bowl and add the boiled butter and egg mixing well to form a dough.
Turn onto a board with a sprinkling of flour and ease into a round of pastry. Roll out large enough to cover the stew in the pie dish.
Place the meat in a large ovenproof pie dish, and cover with the pastry. Brush with egg yolk for a golden finish.
Bake the pie in the oven for 20 minutes until the pastry is crisp and golden.