Chop potatoes evenly, about 2.5cm thick. Place in a pan, rinse, then cover with water adding a generous amount of salt (1 tablespoon per litre).
Bring to the boil then simmer. Cook until soft. Strain the potatoes then leave for a few minutes. Boil the milk with half of the scallion's and season.
Mash the potatoes and add the butter and milk. Add the raw scallions for texture. Keep warm.
Place a pan on the heat until very hot. Season the beef. Add a splash of oil to the pan and sear the beef for 2 minutes each side. Add a touch of butter towards the end of cooking. Remove and rest on a baking tray for 4 minutes.
Cook the wild mushrooms in a pan with butter and garlic and finish with tarragon.
Reheat the sirloin for a minute- add a generous amount of champ, place the mushrooms over the beef and pour the sauce.