Start by making your mushroom duxelles. Peel your onions and garlic and place into a food processor to chop into fine pieces. Heat a large pan with oil and once hot, add in your onions and garlic and stir on medium heat for 5 minutes until soft and translucent.
Whilst the onions and garlic is softening, add your mushrooms into the food processor and chop into fine pieces. Now tip the mushrooms into the pan, add the thyme and stir to combine. Add the brandy/whisky and continue to cook for 5 minutes on medium. The mushrooms will absorb all the liquid, shrink and then release the moisture back into the pan during the cooking process.
Continue to cook on medium to low to evaporate as much moisture as possible. Transfer to a bowl and allow to cool down. Refrigerate if you don’t plan to assemble this the same day.
Now it’s time to sear your beef. Season the tenderloin with salt and pepper all over and place a large pan on high heat with a tablespoon of oil. The pan needs to be super hot so allow the pan to start smoking before placing the beef into the pan to sear.
Sear every side of the beef including the ends for 1-2 minutes maximum. You want as much char and caramelisation on the outside but not really cooking the meat inside at all, as this will be cooked in the oven during the baking process.
Remove from the pan and transfer to a tray. Immediately spread the Dijon mustard all over the beef whilst it is still hot. This prevents the meat from drying out and allows it to absorbs the flavor of the mustard into the meat. Allow to cool, cover and refrigerate if you are not planning to assemble that same day. You can do this up to two days in advance of serving.
Now make your crepe. Add all ingredients into a bowl and whisk to combine until smooth.
Preheat a large pan with a teaspoon of butter and once melted, pour in 1 ladle of the crepe batter, just enough to coat the surface of the pan. Cook on medium for 1-2 minutes before flipping and cooking for another minute before placing onto a plate to cool. The first crepe never works, so now cook a further 2 crepes that you will actually use to roll the beef. Allow to cool, cover and refrigerate if you are making this in advance.
Time to assemble your Beef Wellington! Place 2 of the crepes down onto a chopping board overlapping each other so you have a rough rectangle crepe surface. The length of the crepes needs to be long enough to wrap entirely around the beef wellington. Now place down 5-6 slices of prosciutto down onto the base of the crepe, carefully overlapping each other.
Then carefully and evenly spread the mushroom duxelles all over the prosciutto, making sure there is enough to evenly coat the beef when rolled.
Now place your beef along the base of the mushroom duxelles and gently begin to roll the beef into a ‘burrito’ so that the crepe, prosciutto and mushrooms are completely covering the beef.
Then cover with cling wrap with enough excess wrap on the sides to twist and seal off the ends with cling wrap. Place into the fridge to chill for 1 hour.
Now roll your puff pastry onto your bench top and brush the top with some some egg wash (2 egg yolks mixed with a dash of water). This will help your beef stick to the pastry. Then slowly roll the pastry around the beef until the edges meet. Cut off any excess pastry on the sides and then using your fingers, pinch close the the ends of the pastry, making sure the beef is fully encased in the pastry and there are no holes.
Brush the entire surface of the pastry with the same egg wash from before.
You could now place this into the oven to cook or you can alternatively score the pastry or add a lattice over the top to make it look like a spectacle!
If so, take another sheet of pastry, large enough to cover the exposed surface of the Beef Wellington and using a sharp knife cut vertical slits/dashes down the pastry to create a lattice effect.
Carefully drape the lattice over the current pastry and spread it evenly, cutting off any overhang on the sides.
Finally, gently brush the lattice and pastry again with the same egg wash. Sprinkle a generous amount of flakey salt over the top of the pastry.
Bake in the oven at 220 degrees Celsius for 45 minutes or until the pastry is nice and golden brown.
Remove from the oven and allow to rest for 15 minutes before slicing and serving.