Method

• 1.
  For the butter, combine the cider, vinegar, eschalot, garlic and lemon juice in a medium saucepan. Bring to the boil over high heat. Reduce heat and simmer for 30 minutes or until all liquid has evaporated and eschalot is tender. Set aside to cool.
• 2.
  Place the butter in a stand mixer fitted with the whisk attachment and whisk at high speed for 2-3 minutes until white and fluffy. Fold in lemon zest, eschalot mixture and chopped tarragon. Season to taste. Roll half the mixture into a log, wrap in plastic wrap and freeze to use at a later date. Transfer remaining amount to a small saucepan and set aside.
• 3.
  Prick leeks all over with the tip of a knife. Brush with a little oil and season with salt flakes and freshly ground black pepper. Over a hot charcoal barbecue or a very hot chargrill pan placed over high heat, grill leeks for 8-10 minutes, turning occasionally, until tender when pierced with a toothpick and blackened. Wrap immediately in foil and set aside to steam for 10 minutes. Discard outer blackened layer, trim ends and split leeks in half, lengthways. Drizzle with lemon juice and a little extra oil and set aside, covered, to keep warm.
• 4.
  Brush steak with a little remaining oil and season. Over a hot charcoal barbecue or a very hot chargrill pan placed over high heat, chargrill steak for 6-8 minutes each side, depending on the thickness, for rare (see note). Rest, covered, for 10 minutes.
• 5.
  While the steak rests, reheat the butter. Stir constantly over low heat until almost completely melted (it will look separated). Remove pan from heat and stir vigorously until well combined.
• 6.
  To serve, slice steaks across the grain. Arrange grilled leeks on serving plates, top with sliced steaks and spoon over the tarragon cider butter.