Flatten the beef steaks to tenderize them using a rolling pin or a meat mallet, placing them between two sheets of plastic wrap.
Cut the steaks into strips about 5 mm (about 1/4 inch) thick and 5 cm (about 2 inches) long.
Heat the oil over high heat in a skillet. Sear the beef strips on both sides in small batches, arranging them in a single layer. Refer to the tips above. Each batch takes about 5 minutes to cook.
Remove the beef and transfer it to a plate. Sear the remaining meat the same way, then cover loosely with aluminum foil.
Note: Cut the beef strips slightly thinner than shown in the image below and brown them well for maximum flavor. It is important not to overcrowd the pan and to sear the meat in very hot oil over high heat.
Sear the meat on both sides
Peel and finely chop the onion. You can also slice it thinly if you prefer a more pronounced onion texture.
Add the butter to the skillet and gently sauté the onion and grated garlic over medium heat, without browning them.
Once the onion becomes translucent, add the mushrooms and stir.
Season with pepper and cook for about 2 minutes. The mushrooms will soften and release their moisture. If you are using flour, add it at this stage. Stir well before adding the broth.
Dissolve the beef bouillon cube in hot water to make the broth.
Deglaze the mushrooms with half of the broth.
Add the mustard, paprika, and a small amount of tomato paste, then stir. Adjust the amount of tomato paste depending on how light or deep red you want the sauce to be.
Let simmer until the mushrooms are fully cooked and the sauce has reduced slightly.
If you do not have sour cream, combine the heavy cream with 1 tablespoon of white vinegar in a bowl and mix well until thick and smooth. This mixture can be used as a substitute for sour cream.
Return the beef to the skillet and stir to combine.
Add the cream and stir until everything is well combined. You can also add the cream first, then return the beef to the pan.
As soon as the mixture starts to simmer, add the remaining broth while stirring.
Adjust the amount of sauce to your liking. Let the dish simmer over high heat so the sauce thickens quickly.
If you plan to serve the Beef Stroganoff with pasta, rice, or crushed potatoes, keep a bit more sauce. If serving it on its own, a well coated beef is enough.
Sprinkle with chopped parsley to taste.
Serve immediately, hot, with pasta, crushed potatoes, rice, or French fries. You can also enjoy it on its own.
If you would like to make this dish ahead of time, prepare the recipe without adding the cream, then stir it in just a few minutes before serving.