Place the mushrooms into a large food processor and pulse until they are roughly chopped.
Add the shallots, garlic, thyme, and some salt and pepper, and pulse until you have a finely chopped mixture.
Heat a large non-stick frying pan over medium-high heat, and cook the mushroom mixture (in a dry pan), for about 10 minutes until the moisture in the mushrooms has evaporated as much as possible. It is important that the mixture is as dry as possible to avoid the beef wellington getting soggy later during baking.
When you have finished cooking the mushroom duxelles, remove the mixture to a flat plate or tray, and set aside to cool completely.
Preparing the beef
Heat a large frying pan with some olive oil. The pan should be wide enough to fit the piece of meat comfortably.
Season the beef generously with salt and pepper.
Sear the beef on all sides, including the ends.
Once you have seared the beef, place it on a large plate or tray. (Do not wash the pan – save it for making the Green Peppercorn Sauce later.)
While the beef is still warm, brush it all over with the Dijon mustard.
Leave the beef to cool completely.
Assembling the beef wellington
Place a large sheet of clingfilm on the work surface. The clingfilm needs to be about 20 cm wider than the piece of beef, and long enough to allow you to wrap the beef completely. You may need to overlap pieces of clingfilm to make it wide or long enough.
Layer the slices of prosciutto on the clingfilm, such that they are a bit wider than your piece of beef. The slices should sit vertically in front of you.
Spread the mushroom duxelles in a thin layer over the prosciutto.
Place the beef on top, sitting horizontally in front of you.
Carefully wrap the beef in the prosciutto, using the clingfilm to roll the mixture away from you. Wrap the beef tightly in the prosciutto. Tuck in the ends and place the beef in the fridge for at least 15-30 minutes.
Place another sheet on clingfilm on the work surface. Again, it needs to be about 20 cm wider than the piece of beef (now wrapped in prosciutto), and long enough to allow you to wrap the beef completely.
Roll out of the puff pastry to a size which will be large enough to wrap the beef, leaving a bit of excess on the sides.
Remove the clingfilm from the beef (now wrapped in prosciutto), and place the beef on top of the puff pastry.
Like before, carefully wrap the beef in the puff pastry, using the clingfilm to help you wrap it tightly. Tuck in the ends, and return the wrapped beef to the fridge for at least 30 minutes. At this point, you could leave it in the fridge overnight.
Cooking the Beef Wellington
Preheat the oven to 200°C (390°F).
Brush the Beef Wellington with the beaten egg yolk.
If you wish, you can use the back of the knife to score a pattern in the pastry, taking care not to cut into the pastry. Alternatively, you could decorate the Beef Wellington with some shapes cut from any leftover puff pastry. Stick any pastry decorations onto the Beef Wellington with some egg wash, and also brush the decorations with egg wash.
Place the Beef Wellington on a baking tray lined with baking paper.
Bake for approximately 40 minutes if you like your meat cooked medium/medium-rare. A digital thermometer, inserted in the middle of the beef, should read about 60°C (140°F). If you like your meat more rare, cook it for less time. As a rule of thumb, you should start checking the temperature from about 30 minutes. Bear in mind that the meat will continue to cook at is cools.
If the Beef Wellington is browning too quickly and/or your haven’t reached your desired temperature yet, cover the Beef Wellington with a piece of foil to stop it browning further.
Once the Beef Wellington is cooked to your liking, leave it to rest on a board or tray for at least 15 minutes or more.
To serve, cut the Beef Wellington into thick slices. Serve with Green Peppercorn Sauce (see recipe below) and a side of vegetables, such as roast potatoes, roasted tomatoes, steamed beans, or even a salad.
To make the Green Peppercorn Sauce
In the same pan which you used to sear the beef, add the butter and shallots.
Use a wooden spoon to try and scrape any caramelised bits from the pan.
Add the brandy or cognac and let the mixture bubble away for a few minutes.
Add the stock, cream, and green peppercorns.
Simmer gently for 5-10 minutes.
Meanwhile, mix the cornflour (cornstarch) with a bit of water.
To thicken the sauce, pour some of the cornflour mixture into the sauce and quickly mix it in with a spoon. Add more cornflour mixture as necessary until the sauce has thickened to your liking.
Taste for seasoning.
Serve the sauce hot or warm.
MAKE-AHEAD TIPS I think it is best to start this recipe the night before, or otherwise set aside a whole morning to prepare the Beef Wellington for serving later in the evening. The Green Peppercorn Sauce can also be made ahead of time. Upon reheating, you may have to thin it with some water or cream.
COOKING MEAT PERFECTLY When checking the temperature of the meat with a digital thermometer, take the temperature from the side of the Beef Wellington and in the centre. The temperature of the meat here will be less than at the ends, i.e. the ends of the Beef Wellington generally have meat which is more cooked than in the centre. I tend to aim for medium-rare in the centre, which will give me medium-cooked meat at the ends. Keep in mind that meat continues to cook as it cools, so take your Beef Wellington out of the oven if the thermometer gives you a reading a few degrees below your target.
VARIATIONS If you wish to use a crêpe in place of prosciutto in this recipe, you can make some thin crêpes using my recipe here. Depending on the size of your crêpes, you will need 2-4 crêpes to completely encase the beef. Assemble and overlap the crêpes to form a rough rectangular shape. Spread the mushroom duxelles over the crêpes and proceed with the rest of the recipe.
OVEN TEMPERATURES All recipes on this website state temperatures for a regular oven (i.e. a conventional oven without fan). If you have a convection oven with a fan, please consult the manufacturer’s handbook on how to adjust the temperature and baking time accordingly.