This recipe is from chef Matthew Moran of Moran's Restaurant and Cafe in Sydney. He recommends when buying the beef, choose thick sirloin steaks, grain-fed preferably, and that they be brought to room temperature before cooking
To make the mashed potato, put the potato in a saucepan and cover with cold water. Add salt, if desired, and bring to a boil. Cover and simmer for 20-25 minutes, or until tender. Drain and return the potato to the saucepan. Put the saucepan back over low heat and let the pan and the potato dry for 1 minute. Remove from the heat and mash the potatoes with a potato masher. Add the butter and pepper. Return to low heat. Stirring vigorously with a wooden spoon, gradually add the hot cream (or as much as necessary) until the mixture is light and fluffy.
While the potatoes are cooking, make the salsa verde. Mix together all the ingredients in a food processor and blend to a paste.
Chargrill the sirloins. Make sure the fire has burnt down to very hot embers or turn on the gas barbecue. Rub olive oil and pepper over the steaks. Place on a grill over the fire and cook for about 8-10 minutes on both sides, for a rare steak. Remove to a warm place and allow to rest for 15 minutes.
To serve, place the steak on warm plates and spoon the hot mashed potato next to it. Pour the heated demi-glace over the steak and top with the salsa verde.