Season the beef with salt and pepper. In a large skillet over medium-high heat, melt 2 tablespoons of the butter and add the beef to the skillet, spreading it into an even layer. Cook for 1 minute, then use tongs to toss the beef until mostly browned. Remove to a plate; set aside.
Melt the remaining butter and add the mushrooms and onions to the skillet. Cook, stirring occasionally, until the mushrooms are soft and the onions are translucent, about 5 to 7 minutes.
Sprinkle the mixture with the flour and stir, cooking for 1 to 2 minutes, until no raw flour remains and it emits a nutty aroma. Gradually stir in the beef broth, then add the salt, pepper and paprika. Bring to a simmer and cook until slightly thickened. Add the beef back to the sauce, reduce the heat to low, and cook for 2 or 3 minutes, until the beef is warmed through. Remove from the heat and stir in the sour cream until completely combined.
Serve with cooked egg noodles and a sprinkle of fresh parsley. Leftovers can be stored in the refrigerator for up to 4 days.