Cold rare roast beef with dill and mustard

Ingredients

• 500 g Thick piece of beef fillet, at room temperature
• 175 ml 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