Trim any fat off the steak then cut the meat into strips and season with pepper.
Heat a large, lidded nonstick frying pan and dry-fry the onions and mushrooms for 2-3 minutes over a medium heat.
Pour in the stock or sherry and Worcestershire sauce, cover and simmer for 5 minutes, shaking the pan occasionally. Remove the lid and boil for 2 minutes, until there are only 4-5 tablespoons of liquid left. Lift out the vegetables and keep them warm.
Add the beef to the pan and simmer for 3-4 minutes until browned.
If using the brandy, turn up the heat slightly to warm the base of the pan, pour in the brandy and ignite it. When the flames have died down, stir in the garlic, if using.
Return the onions and mushrooms to the pan and cook for 2 minutes. Stir in the sour cream and parsley and adjust the seasoning to taste. Serve garnished with sprigs of parsley.