By William Lander, Shaun Searley and Daniel Morgenthau
Categorically the best way to use up leftover Sunday roast beef. Don’t be shy with the horseradish: it’s the heat of this together with the rich beef and peppery watercress that make this sandwich such a classic.
Peel the beetroot and use a mandoline to thinly slice the beetroot into a bowl (or jar, if you are intending to use it a few days later). In a small saucepan, bring the pickle liquor to the boil and pour over the beetroot. Leave to one side to cool. This could be prepared a few days in advance: keep the beetroot in a covered jar or bowl in the fridge.
Thinly slice the beef, either using