Slice the steak against the grain into thin, bite-size strips. Season with some salt & pepper.
Add the olive oil and 1 tablespoon of the butter to a large skillet over high heat. Let the pan heat up for a few minutes so it's very hot.
Add the beef in a single layer (just sort of "sprinkle" it into the pan ensuring it's spread out a bit) and don't touch it for a minute. Stir/flip it with cooking tongs and cook it for another minute or so. Leaving it a bit rare at this stage is ideal. Transfer it to a plate.
Turn the heat down to medium-high. Add the rest of the butter to the pan and let it melt, then add the mushrooms and onions. Sauté, stirring occasionally, until the mushrooms' water has been released, cooks off, and the mushrooms and onions soften and get a nice sear.
Reduce the heat to medium. Add the flour and garlic to the pan and stir constantly for about 1 minute.
Add in the Dijon mustard, Worcestershire sauce, and beef broth, and give it a good stir, scraping up any browned bits from the bottom of the pan.
Stir in the heavy cream and sour cream, then add the beef back to the pan (and any plate juices) and cook for a couple of minutes or until the steak is heated through and cooked to desired doneness. Don't let it bubble too much or there's a chance the sauce will curdle. Season with more salt & pepper as needed and serve immediately with fresh parsley sprinkled on top.