Thinly slice the beef against the grain. Season with salt and black pepper.
Heat oil in a large skillet over medium-high heat. Add the beef in a single layer and sear until browned on both sides, working in batches if needed.
In the same skillet, melt the butter. Add the chopped onion and cook until soft and translucent, about 4 minutes.
Add the mushrooms and cook until they release their moisture and turn golden brown.
Sprinkle the flour over the mushroom mixture and cook for 1 minute, stirring constantly.
Gradually whisk in the beef stock and Dijon mustard until the sauce thickens and becomes smooth.
Stir in the heavy cream, sour cream, soy sauce, salt, and black pepper. Bring to a gentle simmer, then reduce the heat and cook for 3 minutes.
Return the beef and any accumulated juices to the skillet. Simmer for 5 minutes, until the beef is tender and heated through.
Remove from heat, stir in fresh parsley, and serve over rice, mashed potatoes, or egg noodles.