Place each steak between 2 sheets of wax paper and use a rolling pin to gently pound them until they're an even ¼ inch (5mm) thick throughout.
Heat the butter and sunflower oil together in a large frying pan over medium-high heat. Once the butter begins foaming, add the pounded steaks to the pan and cook over high heat for about 3 minutes on each side until they develop a nice brown crust.
Remove the cooked steaks from the pan using tongs or a spatula and transfer them to a plate. Cover loosely with foil to keep them warm while you prepare the sauce.
Pour the brandy into the same pan and add the onion. Cook over high heat, stirring occasionally, for a few minutes until the onion softens and absorbs most of the brandy, creating a fragrant base for your sauce.
Stir in the stock, Worcestershire sauce, lemon juice, and parsley. Season with salt and pepper to taste and let the sauce cook for about 2 minutes to meld the flavors together.
Return the steaks to the pan and spoon the rich sauce over them. Heat briefly for just a minute to warm the steaks through, then serve immediately while piping hot.