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Prepare to the end of step 3 a day ahead and chill or freeze. Allow an extra 10-15 mins when cooking from chilled (and defrosted).
Heat 1 tablespoon of oil in a large heavy-based saucepan or casserole dish. Season the beef chunks and fry in batches over a high heat, until browned all over. Remove each batch to a plate while you brown the remaining beef, with a little more oil as needed. When all the meat is browned, tip it all back in the pan and stir in the flour. Pour over the stock, add 2 thyme sprigs and bring to a simmer. Cover and cook very gently for 2-2½ hours, until the beef is really tender. If you prefer, you can cook the casserole in the oven at 140°C, fan 120°C, gas 1, for the same length of time.
Meanwhile, melt the butter in a frying pan over a gentle heat. Add the onion and cook for 5 minutes until softening. Add the garlic, mushrooms and the remaining thyme sprig. Turn up the heat and cook until the mushrooms have softened and turned golden. Set aside.
Once the beef is cooked, stir through the mustard, Marmite and sugar, followed by the onions and mushrooms. Spoon into a 1.5-litre pie dish and leave to cool before adding the pastry lid. Brush the rim of the dish with beaten egg and lay the pastry on top, trimming to fit. Crimp the edges if you like, and make a small steam hole with a sharp knife.
Preheat the oven to 200°C, fan 180°C, gas 6. Brush the pie with the remaining beaten egg. Bake for 25-30 minutes until golden brown and piping hot in the centre. Serve with greens.