For the onion, beer and mustard gravy, melt the butter in a pan until foaming, then stir in the sugar. Add the finely chopped onions and cook over a low heat for 20-30 minutes, or until golden-brown and caramelised.
Stir in the mustard powder and pour in the stock and beer and simmer for 15-20 minutes, or until the liquid has reduced in volume by half. Season, to taste, with salt and freshly ground black pepper and keep warm.
For the pancakes, season the flour with salt and freshly ground black pepper and mix with the beaten egg until well combined. Gradually add the milk, whisking continuously, until the mixture comes together as a smooth batter with the consistency of double cream. Add the melted butter and beat until smooth. Stir in the chopped herbs and set aside to stand for 20 minutes.
Heat the olive oil in a non-stick pan and add a few tablespoons of the pancake batter, spreading it out into a circle. Cook for 1-2 minutes, or until golden-brown underneath, then carefully flip over and cook for a further 1-2 minutes, or until golden-brown all over and cooked through. Repeat with the remaining pancake mixture to make 2-3 pancakes, adding more oil to the pan as needed.
Cut the pancakes into squares, lay them flat on an oven tray lined with greaseproof paper and set aside.
For the duxelles, melt the butter in a frying pan until foaming, then add the shallots and fry for 3-4 minutes, or until softened. Add the mushrooms and cook with the lid on for 4-5 minutes, or until tender.
Drain the soaked porcini mushrooms and finely chop, then add to the pan with the mushrooms. Stir in the cream and cook with the lid off for 4-5 minutes, or until most of the liquid has evaporated and the mixture is thick and creamy.
Set aside to cool. Once cooled, stir in the parsley and the egg white. Transfer the mixture to a food processor and blend until the mixture forms a coarse paste.
Spread the mushroom duxelles in an even layer over the pancakes, then cover the tray with cling film and chill in the fridge until the duxelles have set.
For the beef Wellington, preheat the oven to 180C/350F/Gas 4.
Season the beef with salt and freshly ground black pepper. Heat a frying pan until very hot, then add the vegetable oil. Sear the beef all over for 2-3 minutes, turning occasionally, until the beef is golden-brown all over. Set aside to cool.
Roll out the pastry to a 20cm x 30cm/8in x 10in rectangle and place the duxelles-lined pancakes over the pastry. Lay the seared beef fillet in the middle, then draw the pastry and pancakes over the beef, wrapping it completely. Seal the ends of the pastry with some of the beaten egg.
Cut a few holes into the top of the pastry to allow steam to escape. Brush the pastry all over with the remaining beaten egg, then place the beef Wellington on a baking tray. Bake the Wellington in the oven for 25-30 minutes (for rare beef), or until the pastry is golden-brown and crisp. Keep warm.
For the goats' cheese, garlic and chervil mash, melt the butter in a pan and add the lightly crushed garlic. Remove from the heat and allow to infuse for 30 minutes.
Cook the potatoes in a pan of boiling salted water for 10-12 minutes, or until tender, then drain the potatoes and return to the pan. Place back over the heat to drive off any excess moisture, then mash with a potato masher until smooth.
Beat in the cream, goats' cheese and garlic-infused butter, then fold in the chopped chervil. Season, to taste, with salt and freshly ground black pepper.
For the broad beans, lettuce and shallots, melt the butter in a pan and fry the shallots for 3-4 minutes, or until softened.
Add the blanched broad beans and chicken stock to the pan and bring to a simmer. Stir the shredded Baby Gem lettuce into the pan and cook for 1-2 minutes, or until tender.
To serve, thickly slice the beef Wellington and divide among serving plates. Spoon the goats' cheese, garlic and chervil mash alongside, followed by the broad beans, lettuce and shallots. Drizzle over the onion, beer and mustard gravy and serve immediately.