For the bearnaise sauce place the white wine vinegar, water, shallots, peppercorns and tarragon stalks into a small saucepan over a medium heat and simmer until you have a tablespoon of liquid remaining. Strain the liquid into a bowl and set aside.
Clarify the butter by melting it in a small saucepan on a low heat and allow to separate. Skim off the froth with a slotted spoon. Pour the clear butter into a bowl leaving behind the white solids. Keep warm until needed.
Place the egg yolks, tarragon and vinegar reduction into another bowl and place over a saucepan of simmering water. Whisk until thickened and doubled in quantity.
Remove from the heat. Slowly whisk in the clarified butter until you reach a lovely thick consistency. Add enough lemon juice, chopped tarragon and seasoning to taste. Keep warm until needed.
Pre-heat the oven to 220°C.
For the Yorkshire batter, in a bowl, sift the flour and add the salt.
In a separate bowl, thoroughly whisk the eggs, egg whites and milk.
Pour into the flour, whisking continuously until you have a smoother batter. Strain through a sieve. Using a 25cm x 35cm inch (4cm deep) rectangle baking tray, add a thin layer of oil and heat until smoking.
Pour in 3/4 of the batter, place in the oven and cook for 15 minutes.
Keep the remaining batter for the onion rings.
Turn the oven down to 190°C and cook for a further 20 minutes.
Remove from the oven, leave to cool for 5 minutes and then pop onto a chopping board. Cut into 6 equal pieces. Brush the steak with oil and season with salt and pepper.
On a hot griddle or frying pan, fry the steak for three minutes on each side (or longer if you like it well done). Leave the rest for at least five minutes. Slice into 1cm strips.
On the Yorkshire, place a layer of watercress and rocket, followed by the sliced steak and spoon over the bearnaise sauce.
Heat the oil to 180°C. To make the onion rings, dust in the plain flour and then dip in the remaining batter, gently place into the oil. Cook for approximately 1-2 minutes on each side until golden and crispy.
Season the onion rings and serve alongside the yorky open sandwich.