Preheat the oven to 180°C.
Pierce the pork tenderloin and insert rosemary sprigs, then season with salt and pepper.
Sear the pork in olive oil in an ovenproof pan for 3 minutes until golden.
Add butter to the pan and roast in the oven for 12 minutes until cooked through.
Rest the meat on a warm plate.
Boil the potatoes for 8 minutes until tender.
Drain and crush the potatoes with butter, chives, and seasoning.
Simmer the cream in a small pan for 5 minutes until reduced by one third.
Stir the mustard and lemon juice into the reduced cream.
Slice the pork and serve over the potatoes with the sauce drizzled around.