Cut the sirloin steak into thin strips and season it with salt and pepper.

Heat the olive oil and 1 tablespoon of butter in a heavy skillet over medium high heat. Add the steak strips in a single layer, and let them cook undisturbed for about 1 minute to get a good sear.

Flip them and cook until browned, about another 2 minutes. Remove the steak from the pan and transfer it to a separate plate.

Add the remaining 2 tablespoons of butter to the skillet. Add the onions and sauté for 2 to 4 minutes, or until the onions start to turn translucent.

Add the mushrooms, season them with salt, and cook for 5 minutes, or until the mushrooms and onions are cooked and nicely browned.

Add the garlic and cook for another minute or until fragrant.

Add the brandy (or other alcohol) to deglaze the pan, making sure to scrape up the browned bits off the bottom. Let the mixture cook for another 2 to 4 minutes until the liquid has
reduced by about a third.

In a large bowl, whisk together the beef broth, soy sauce, mustard, and cornstarch (or flour) until smooth. Set aside.

Add the beef broth mixture to the skillet, and stir to combine. Bring the mixture to a simmer and cook for 5 minutes, stirring occasionally, until the sauce has thickened slightly.

Turn the heat to low, and stir in the sour cream (or crème fraîche/Greek yogurt) until combined.

Return the cooked steak to the pan and cook for 2 to 3 minutes, or until the steak is heated through. Taste and adjust seasoning with additional salt and pepper, if needed.