Heat olive oil or butter in a large skillet over medium-high heat.
Add the sliced beef to the skillet and cook until browned on all sides.
Remove the beef from the skillet and set it aside.
In the same skillet, add chopped onion and cook until softened.
Stir in the minced garlic and cook briefly until fragrant.
Add the sliced mushrooms and sauté until they become tender.
Pour in the beef broth and bring the mixture to a gentle simmer.
Stir in the heavy cream and Dijon mustard.
Add paprika, salt, and black pepper to season the sauce.
Return the cooked beef to the skillet and mix well.
Simmer the mixture for several minutes until the sauce thickens slightly.
Lower the heat and stir in the sour cream until the sauce becomes smooth and creamy.
Cook for another minute until everything is heated through.
Garnish with chopped parsley before serving.