Preheat the oven to 180C/350F/Gas 4.
For the pork, make two incisions in the tenderloin with the tip of a sharp knife and poke the rosemary sprigs into the holes. Season well with salt and freshly ground black pepper.
Heat the olive oil in an ovenproof frying pan and sear the meat all over for 2-3 minutes, or until golden-brown. Add the butter to the pan, then transfer to the oven to roast for 10-12 minutes, or until the pork is cooked through. (The juices will run clear when the pork is pierced in the thickest part with a skewer.) Remove from the pan and allow to rest on a warm plate.
For the potatoes, cook the potatoes in a pan of boiling water for 7-8 minutes, or until tender, then drain and crush lightly, adding the butter and chives. Season, to taste, with salt and freshly ground black pepper.
For the sauce, place the cream into a small pan and simmer for 4-5 minutes, or until reduced by about a third in volume, then stir in the mustard and lemon juice.
To serve, spoon the crushed potatoes onto a serving plate. Thickly slice the pork and arrange on top of the potatoes, then spoon the sauce around the edges of the plate.