Season the beef with salt and freshly ground black pepper. Heat a large pan until hot, then add the vegetable oil and fry the beef for 2-3 minutes, turning occasionally, until browned all over.
Add the smoked bacon and fry for a further 1-2 minutes, then add the onion, carrots, garlic and mushrooms. Fry for 2-3 minutes, then add the flour and stir well to prevent any lumps from forming. Cook for a further minute.
Pour in the dark ale, bring to the boil and cook for 8-10 minutes, or until reduced by half. Dissolve the chicken bouillon powder in the water, then add to the pan along with the bouquet garni, wholegrain mustard and new potatoes. Bring to the boil and season with salt and freshly ground black pepper, then reduce to a simmer and cook for two hours, or until the beef is tender.
Add the chopped tarragon to the pan and season, to taste, if necessary. Remove the pan from the heat and set aside to cool. Discard the bouquet garni.
Preheat the oven to 200C/400F/Gas 6.
Spoon the beef mixture into a 20cm x 15cm/8in x 6in pie dish and add the oysters, folding them gently into the beef mixture.
Roll out the puff pastry until it is 3mm thick, then cut out pieces of pastry large enough to fit over the top of the pie dishes as a lid, with a bit of overhang. Crimp the edges with your fingers, securing the pastry to the dish.
Prick the surface of the pastry with a fork, then brush with the beaten egg. Place the pies onto a baking tray and bake in the oven for 20 minutes, or until the pastry is golden-brown and crisp.
Serve the pies with sides of lightly boiled seasonal green vegetables, such as green beans, broccoli or spinach.