* Halve the potatoes, quartering larger pieces, and pop onto a lined baking tray.
* Drizzle with oil and season with salt and pepper.?Spread out in a single layer.
* Roast on the top shelf of the oven until fork tender,?25-35 mins.
Get Prepped
* Meanwhile, halve, peel and chop the shallot into small pieces.
* Pick the tarragon leaves from their stalks and finely chop (discard the stalks).
* Take one egg (per 2P) and separate the yolk from the white (you can use the egg white later).
TIP: You'll be using the egg yolk raw, so use very fresh eggs to make the sauce.
Make the Sauce
* Place a small pot over medium-high heat with 2 tbsp butter (per 2P).
* Add the shallot and cook for 2-3 mins.
* Add the tarragon and cook for 2 mins more.?Remove the pot from the heat.
* Meanwhile, make a dressing with half the vinegar and a drizzle of oil.
* Just before serving toss the salad leaves with the dressing and season to taste with salt and pepper.
Fry the Beef
* Place a large pan over high heat with a drizzle of oil.?Season the beef with salt and pepper.
* Once hot, fry the beef until browned, 1-2 mins each side for medium-rare.
* Cook for another 1-2 mins on each side if you like it medium and a further 1-2 mins on each side if you want it well-done. IMPORTANT: Wash hands and equipment after handling raw meat and its packaging. Meat is safe to eat when the outside is browned.
* Once cooked, transfer to a board and allow to rest.
Finishing Touches
* Add the egg yolk, half a mustard sachet (per 2P) and remaining vinegar to the pot with the shallot.
* Return the pot to low heat (as low as possible so as not to cook the egg yolk and mix vigorously for 2-3 mins until the sauce thickens.
* Remove from the heat and stir in 1 tbsp water (per?2P) and a good drizzle of oil.
* Add a splash of water the sauce is too thick, you want a smooth sauce that coats the back of a spoon.?Season to taste with salt and pepper.
Serve and Enjoy
* Thinly slice the beef.
* Divide the potatoes and sliced beef between plates.
