For the sour cream mash, place the potatoes in a saucepan of cold salted water, bring to the boil and cook for 10 minutes or until tender. Drain, then mash well. Stir in sour cream, season with sea salt and freshly ground black pepper, then keep warm.
Meanwhile, season beef, then, using a meat mallet or rolling pin, flatten out slightly. Heat oil in a large frypan over high heat. In 2 batches, cook the steaks for 2 minutes each side for medium-rare or until cooked to your liking. Remove the steaks from the frypan, loosely cover with foil, then set aside to rest while you make the sauce.
Return the frypan to medium heat and add the butter. Once the butter has melted, cook the eschalot, stirring, for 1 minute, then add the garlic and mushroom, and stir to combine. Add brandy and cook for 2-3 minutes until liquid almost evaporates. Add the Vegemite, mustard and stock and cook, stirring, for 2 minutes. Add the cream and cook for a further 2 minutes or until slightly reduced. Season with sea salt and freshly ground black pepper.
To serve, divide sour cream mash and steaks among serving plates and drizzle over sauce. Sprinkle with some chopped parsley and serve with rocket.
