Place porcini mushrooms in a small heatproof bowl. Cover with the boiling water. Set aside for 20 mins to soak.
Meanwhile, heat half the oil in a large frying pan over medium heat. Add the onion and cook, stirring, for 5 mins or until onion softens. Add the garlic and stir for 1 min or until aromatic. Transfer to a heatproof bowl.
Heat half the remaining oil in the pan over high heat. Add brown and cup mushroom. Cook, stirring occasionally, for 5 mins or until the mushroom is golden brown and just tender. Add to the onion in the bowl.
Heat the remaining oil in the pan. Season the steaks well. Add to the pan and cook for 2-3 mins each side for medium-rare or until cooked to your liking. Transfer to a plate and loosely cover with foil.
Add the brandy, wine or stock to the pan and stir with a wooden spoon, scraping the base of the pan. Cook for 2 mins or until the liquid reduces by half. Add the porcini mushrooms and liquid and bring to the boil. Cook, stirring occasionally, for 4 mins or until the mixture reduces by half. Add the onion mixture, paprika, tomato paste, mustard, Worcestershire sauce and stir to combine. Cook for 2 mins or until the mixture is well combined and heated through. Add the sour cream and cook, stirring, for 2-3 mins or until the sauce boils and thickens slightly. Remove from heat. Add the steaks and turn to coat in the sauce.
Season and serve immediately with steamed green beans.