Marinate the meat
Coat with mustard, season.
Brown the meat
Heat the oil in a non-stick frying pan. Brown the meat all over for approx. 10 mins. over a medium heat.
Roast in the oven
Place the meat in an ovenproof dish. Wipe out the frying pan, set aside.
Roast for approx. 70 mins. in the centre of an oven preheated to 180°C.
Finely chop the shallot. Heat the butter in the frying pan and sauté the shallot. Add the rosemary.
Pour in the wine and stock, bring to the boil. Stir the cornflour into water and add to the pan, stirring with a whisk. Reduce for approx. 10 mins. to a fairly thick jus, strain if necessary, season.
Cut the potatoes into pieces, place in a pan of boiling salted water, cover and cook over a medium heat for approx. 20 mins. until soft. Drain the water.
Allow the residual moisture on the potatoes to evaporate by shaking the pan on the switched-off hob until the potatoes are dry and whitish in colour.
Push the potatoes straight into the pan through a sieve. Stir in the butter. Gradually add the milk, stirring constantly. Return the pan to the warm hob a few times. Stir briefly and vigorously until the potato mixture is fluffy.
Stir in the mustard, season.
Remove the meat from the oven, cover and leave to rest for approx. 10 mins. before carving.