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Leave the beef at room temperature for 1 hr before cooking. For the dauphinoise, heat the butter in a saucepan over a medium heat. Once foaming, add the shallots and a pinch of salt and fry for 10 mins or until softened. Add the vinegar and reduce it by half, then pour in the cream and milk. Tip in the garlic and tarragon stalks and bring to a simmer. Take off the heat and set aside to infuse for 30 mins.
Tip the mixture into a bowl, scoop out the garlic and tarragon stalks, and discard. Whisk through most of the chopped tarragon, the egg yolk and some seasoning. Mix the potato slices through the infused cream until they are fully coated, then arrange in overlapping layers in a buttered 30cm round dish. Pour over any remaining liquid, scatter with the gruyère and set aside.
Heat the oven to 220C/200C fan/gas 7. Heat the olive oil in a large non-stick pan over a high heat. Add the beef and fry for 5-7 mins, on all sides, until the fat is golden brown. Remove and put in a roasting tin. Bash the peppercorns and salt together with a pestle and mortar, and rub this over the fatty parts of the meat. Cut the garlic bulb in half through the cloves, then add to the tin.
Turn the oven down to 180C/160C fan/gas 4. Put the beef in the oven to cook for 55 mins-1 hr 5 mins (for medium-rare). Put the dauphinoise on the lower shelf to cook for the same amount of time, then add the carrots to the roasting tin for the last 25 mins. A cooking thermometer should read 55C when the beef is cooked. Cover with foil and leave to rest for 25 mins.
Make the dressing by whisking all the ingredients together with some seasoning, then toss with the watercress. Serve the beef thinly sliced with the dauphinoise, carrots, the roasted garlic and the lightly dressed watercress leaves.