Herby dijon roast beef with rosemary potatoes

Method

1. Heat the oven to 180ºC fan/gas 6. Put the potatoes in cold salted water, bring to the boil and simmer for 15-20 minutes. Drain, leave to cool a little, then peel while still hot. Leave to cool completely (see Make Ahead).
2. Heat a dry frying pan with 1 tbsp oil. Pat the beef dry, sear it on all sides, then put on a baking tray. Season, then brush the mustard over the top and sides. Press half the herbs lightly onto the mustard, then roast for about 25 minutes for medium rare. Remove, cover with foil and rest for 10 minutes (the internal temperature after resting should be 55-60ºC; it will rise in temperature while resting).
3. Meanwhile, for the remoulade, mix the yolks with the mustard and a pinch of salt. Whisk in 1 tbsp of the oil, drop by drop, until the mixture starts to thicken and emulsify, then whisk in the remaining oil in a thin trickle. Mix in the crème fraîche, cornichons, capers and cornichon vinegar. Stir in the tarragon, lemon zest and juice, then season. Cover and chill in the fridge (see Make Ahead).
4. Heat the remaining 1 tbsp olive oil in the pan used to sear the beef, then stir-fry the potatoes for 8-10 minutes until crisp. Add the rosemary sprig after 6 minutes. Season.
5. Sprinkle the remaining herbs over the meat, then serve with the potatoes, remoulade and your favourite seasonal greens.