Lucas Hollweg's Cold Rare Roast Beef With Dill And Mustard

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Ingredients

500 g (17.6oz) thick piece of beef fillet, at room temperature
175 ml (6.2fl oz) flavourless vegetable oil (sunflower, groundnut) plus a splash extra
1 pinch salt
1 pinch pepper
4 tbsp dijon mustard
1.5 tbsp red wine vinegar
2 tsp caster sugar
1 splash of hot water
5 tbsp chopped dill
2 handfuls watercress
1 tbsp extra virgin olive oil