Heat oil in large frying pan, colour the lamb fillets quickly then take out. Cut potatoes in half lengthwise and put cut side into the pan, meanwhile brush lamb fillets with mustard and then lay on potatoes. Place the tomatoes into the pan, season all with salt and pepper then put in oven 200c for 10 minutes
Take meat out then potatoes and rest while you make the sauce. Throw out excess oil and place the pan back on the heat, add balsamic and reduce a little then take out the tomatoes and add the stock, butter and parsley and bring to the boil, simmer for a minute or so
Put tomatoes and potatoes on plate. Arrange lamb on top and finally pour sauce over and serve