Slice the beef into thin strips (freezing the beef for 30 minutes ahead of time makes slicing much easier).

Pre-heat a large sauté pan over medium-high heat. Add 1 tablespoon of butter and 1 tablespoon of olive oil to the sauté pan and brown the beef in batches, setting it aside when it is nicely browned.

Add the remaining butter and oil to the pan and sauté the onion for a couple minutes. Add the mushrooms and thyme and continue to sauté until nicely browned – another 5 minutes or so. Be careful not to burn the butter. Add the brandy and using a wooden spoon, scrape up any brown bits that have formed on the bottom of the pan and bring to a simmer. Add the beef stock and simmer for a minute or two. Return the beef to the pan, along with any resting juices. Simmer for a minute or two, just to reheat the beef and then remove the pan from the heat.

Stir in the sour cream and mustard. Season to taste with salt and pepper. Serve over egg noodles and sprinkle with a little paprika and chopped fresh dill, if desired.