Preheat the oven to 200C/400F/Gas 6.
For the steak with tarragon butter, mix the balsamic vinegar, tarragon and 175g/6¼oz of the softened butter in a bowl until well combined. Season, to taste, with salt and freshly ground black pepper.
Spoon the butter mixture onto a large piece of cling film. Roll the butter up in the cling film into a cylinder shape, twisting the ends to secure. Chill in the fridge for 1-2 hours or until set.
Once the tarragon butter has chilled, remove it from the fridge and slice into 0.5cm/¼in slices.
Season the steaks, to taste, with salt and freshly ground black pepper.
Heat the remaining butter in an ovenproof frying pan until foaming and fry the steaks, on one side, for 2-3 minutes, or until golden-brown. Top each steak with a couple of slices of the tarragon butter and transfer to the oven to cook for 5-6 minutes (for medium), or until cooked to your liking. Remove from the oven and set aside on a warm plate to rest for 5 minutes.
Meanwhile, for the chips and onion rings, heat a deep-fat fryer to 160C/320F. Lower the potatoes into the deep-fat fryer and cook for 2-3 minutes, or until tender. Using a slotted spoon, remove the chips from the fryer and increase the heat to 190C/375F.
Lower the par-cooked chips back into the deep-fat fryer and cook for 2-3 minutes, or until golden-brown and crisp and cooked through. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper. Keep warm.
For the onion rings, sprinkle the flour into a bowl and pour the milk into a seperate bowl. Dip the onion rings first into the milk, then dredge them in the flour and carefully lower them into the deep-fat fryer. Fry for 2-3 minutes, or until golden-brown and crisp.
To serve, mix the watercress leaves and olive oil together until well combined and season, to taste, with salt and freshly ground black pepper. Place a rump steak onto each of 4 serving plates, top with some onion rings and spoon the chips and dressed watercress alongside.
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