Heat a large 12-inch skillet over medium-high heat and add the olive oil.

Season the beef strips with 1 teaspoon of the salt and 1/2 teaspoon of the pepper. Add the beef to the hot pan and sear for 1 to 2 minutes, then turn with tongs onto the second side. Continue to sear the meat on the second side for another minute, or until golden brown. Transfer the beef to a large plate.

Return the skillet to the heat and add 3 tablespoons of the butter to the pan. When the butter has melted, add the mushrooms and sauté, stirring often, until the mushrooms are well-browned, about 7 minutes. Add the onion to the pan and cook, stirring often, until the onion is very soft and lightly browned around the edges, about 4 to 5 minutes. Add the tomato paste to the pan and cook, stirring, for 1 minute. Whisk in the flour and cook, stirring, for another minute. Add the beef broth and beef to the pan and bring the contents of the pan to a boil while whisking constantly.

Remove from the heat, then whisk in the sour cream and mustard. Set aside, covered and off the heat, while you prepare the noodles.

Bring a large pot of water to a boil, add the remaining 1 tablespoon of salt, and cook the noodles until al dente, about 7 to 8 minutes. Drain the cooked noodles in a colander in the sink and return them to the pot. Add the remaining 2 tablespoons of butter to the noodles and season to taste with black pepper. Toss to blend.

Reheat the Beef Stroganoff gently over medium heat, stirring frequently. Add the parsley and stir to combine. Do not allow the sauce to boil.

Once the beef is hot, divide the noodles among 4 dinner plates and top with the sauce. Serve immediately.