Beef wellington with spinach and chestnut mushrooms

Method

1. Drizzle some of the olive oil over the beef and season with salt and pepper. Heat a wide, deep frying pan over a high heat, then sear the beef all over until browned. Remove from the pan, put on a chopping board and rub all over with the horseradish and English mustard. Set aside to cool.
2. In the same pan used to cook the beef, melt the butter and, once foaming, add the chopped chestnuts and mushrooms. Fry over a high heat, stirring, until the liquid from the mushrooms has evaporated (be prepared: this will take around 20 minutes). Add the brandy (if using), then cook over a high heat for a few more minutes until all the liquid has evaporated again. Tip the mixture into a large bowl and stir in the sage, breadcrumbs and spinach. Check the seasoning, adding salt and pepper to taste. Leave to cool.
3. Beat 1 egg in a small bowl with 1 tbsp milk. Add a drizzle of oil to a 20cm non-stick frying pan, then add the egg mixture and swirl around the pan to make a thin omelette. Set aside. Repeat with 2 more eggs/ milk, so you have 3 thin omelettes. Whisk the final egg in the bowl to use as a glaze for the pastry.
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4. Once the mushroom mixture (duxelles), beef and omelettes have cooled, pat the duxelles generously all over the beef fillet to fully coat it (see Know-how). Roll out the pastry on a floured surface to form a large rectangle. Make sure it’s long enough for the beef to sit lengthways in the middle with enough spare pastry around it to wrap the fillet.
5. Put 2 of the omelettes, slightly overlapping, on the pastry, then sit the beef lengthways on top. Drape the final omelette over the beef (these omelettes will soak up any of the resting meat juices and help prevent your pastry from becoming soggy). Fold one long side of the pastry over the meat, then brush the pastry top with a little beaten egg. Tuck in the short edges, then fold over the other side of the pastry to fully encase the meat. Trim off any excess pastry as you work and reserve it to make decorations – press these onto the pastry, using the egg wash to help them stick. Carefully transfer the beef wellington, seam side down, to the lined baking tray, then brush all over with beaten egg. Chill for 30 minutes (see Make Ahead).
6. Heat the oven to 210°C/190°C fan/ gas 61⁄2. Glaze the pastry again with the remaining beaten egg, then bake the wellington for 30-35 minutes until the pastry is golden brown and puffed. If you have a digital thermometer, push it into the centre of the wellington. It should read 52-56°C for rare. For medium, cook for 35-40 minutes (or 57-59°C) and for well done for 50 minutes (60-63°C). Rest the wellington for 10 minutes, then slice and serve with vegetables.