Bring a large pot of salted water to a boil. Add 8 ounces of wide egg noodles and cook according to package instructions until al dente—usually about 7 to 9 minutes. Drain the noodles and set aside, reserving about ½ cup of the pasta water for later.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Once the oil shimmers, add the thinly sliced 1 pound of beef sirloin in batches to avoid overcrowding. Sear for about 1 to 2 minutes per side until browned but not cooked all the way through. Remove the beef from the skillet and set aside.
In the same skillet, reduce heat to medium. Add the finely chopped medium onion and sauté until translucent, about 3 to 4 minutes. Stir in the minced garlic and cook for another 30 seconds until fragrant. Add 8 ounces of sliced mushrooms and cook until they release their moisture and start to brown, about 5 to 6 minutes.
Pour in 1 cup of beef broth, scraping any browned bits from the bottom of the skillet with a wooden spoon. Add 1 tablespoon each of Worcestershire sauce and Dijon mustard, stirring to combine. Let the mixture simmer gently for 3 to 4 minutes to reduce slightly and deepen in flavor.
Lower the heat to medium-low and return the seared beef to the skillet. Stir in 1 cup of sour cream, mixing until the sauce is smooth and creamy. If the sauce feels too thick, add a splash of the reserved pasta water to loosen it. Season generously with salt and freshly ground black pepper to taste. Allow everything to warm through for another 2 to 3 minutes, but do not boil to prevent the sour cream from curdling.
Add the drained wide egg noodles to the skillet and gently toss until the noodles are coated in the creamy beef and mushroom sauce. Sprinkle chopped fresh parsley on top for a burst of color and freshness. Serve immediately and enjoy!