Add 3 tbsp olive oil to large heavy bottomed saucepan over a medium high heat. Add the braising steak and oxtail to the pan and turn occasionally until the meat is browned. Remove the meat and any juices from the pan and set aside while you cook the vegetables.
Add 2 tbsp olive oil to the same pan and add the sliced onions, carrots, leeks and chestnut mushrooms. Sweat down for about 5 - 10 mins, or until the vegetables have softened, stirring regularly.
Add the garlic cloves and 1 tsp of salt and cook for a further 2 minutes before adding the meat and juices back to the pan with the plain flour. Stir until the flour is combined and cook for a further two minutes to cook out the flour.
Add the bay leaves, thyme leaves, dried mushrooms, tomato purée, mustard powder, balsamic vinegar, Worcestershire sauce, Marmite, stock, ale and a good grind of black pepper. Stir to make sure everything is combined.
Cover with a lid and cook on a medium-low heat for at least four hours, or until the meat is falling off the bones and the sauce has reduced by half. Make sure you stir occasionally so that the mixture doesn’t stick to the bottom of the pan.
Taste and adjust the seasoning as needed. Remove the bay leaves and the oxtail bones, shred any of the meat from the bones and add the meat back to the pan.
Crumble the blue cheese and add to the pan with the butter. Turn off the heat.
Preheat the oven to 200C/180C Fan/Gas mark 6.
Add the filling into a pie or casserole dish. Or you can divide the mixture into individual ramekins if you’re making mini pies.
Brush the beaten egg around the edges of the dish. Unwrap your puff pastry sheet and drape over the the top of the dish, trimming off any excess pastry from around the edges. You can use any leftover pastry for decoration. Brush the top of the pie with the rest of the beaten egg and make a small hole in the middle of the pie to allow any steam to escape.
Bake the pie in the oven for 30-40 minutes, or until the top is evenly golden brown. If you’re cooking individual pies you might want to give them slightly less time in the oven. Leave the pie to stand for a few minutes before you serve it. This would go great with creamy mash and some buttered winter greens.