Pour the oil into a large frying pan over medium heat, add the chopped onion and fry for a few minutes until softened.
Add the chopped garlic and mushrooms and fry for 5 minutes or so, adding more oil if it’s all absorbed.
Add the thyme and bay leaves to the pan followed by the stout, mustard and yeast extract. Turn the heat to low, put a lid on the pan and simmer for around 20 minutes.
Meanwhile, make your pastry. Sift the flour and salt into a mixing bowl and stir in the suet. Gradually stir in enough cold water to make a soft pastry; you’ll need around 200 ml of water.
Set aside one third of the pastry to make the pudding lid. On a floured surface, roll out the rest into a circle big enough to line your pudding basin. Oil the inside of the basin then ease the pastry in to line it. Trim it to 1 cm below the rim of the dish, to allow room for it to rise.
Roll out the remaining pastry and trim it to fit snugly on top of the pastry edge.
Stir the beef-style pieces into your filling mixture, then season it to taste.
Pour the mixture into the pastry case, rub the top edge with a little water then seal the lid on top.
Cover your pudding basin first with a layer of greaseproof paper, then with foil, folding a large pleat in the top. Secure these by tying some string around the circumference of the basin.
Place a saucer or cookie cutter in the base of a large saucepan and sit the pudding on top of this. Half-fill the pan with water and bring it to the boil. Cover the pan with a lid and leave it to simmer gently for two hours, topping up the water if necessary.
Remove the saucepan from the heat and carefully lift the basin out using oven gloves. Leave it to stand for 5 minutes, then remove the string, foil and baking paper.
Loosen the sides of the pudding with a knife, then invert the pudding carefully onto a serving plate large enough for the filling to ooze into.
Carefully slice it up and serve with mashed potato or chips and seasonal veggies.