Bring a large pot of salted water to a boil and cook the egg noodles according to package directions until just al dente. Drain and set aside.
While the noodles cook, pat the beef sirloin strips dry with paper towels and season them with a pinch of salt and black pepper.
Heat 1 tablespoon of olive oil in a large, deep skillet over medium-high heat. Add half of the beef strips in a single layer and sear for 1–2 minutes per side until browned but not fully cooked through. Transfer the beef to a plate and repeat with the remaining 1 tablespoon of olive oil and beef. Set the browned beef aside.
In the same skillet, reduce the heat to medium and add the unsalted butter. Once melted, add the sliced yellow onion and cook for 3–4 minutes, stirring occasionally, until softened and lightly golden.
Add the sliced cremini mushrooms to the skillet with the onions and cook for 5–6 minutes, stirring occasionally, until the mushrooms release their moisture and turn deeply browned.
Stir in the minced garlic and cook for 30 seconds, just until fragrant, being careful not to let it burn.
Pour in the low-sodium beef broth, Worcestershire sauce, and Dijon mustard. Sprinkle in the dried thyme, dried oregano, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Stir well, scraping up any browned bits from the bottom of the skillet.
Bring the mixture to a gentle simmer and let it cook for 5–6 minutes, allowing the flavors to meld and the sauce to reduce slightly.
Return the seared beef strips and any accumulated juices on the plate to the skillet. Stir to coat the beef in the mushroom sauce and simmer for another 3–4 minutes, just until the beef is cooked through and tender.
Reduce the heat to low. Add the cooked egg noodles to the skillet, gently tossing them with the beef and mushrooms until the noodles are well coated in the sauce.
Stir in the sour cream until fully incorporated, creating a creamy, silky sauce that clings to the beef, mushrooms, and noodles. Taste and adjust seasoning with additional salt and black pepper if needed.
Sprinkle the chopped fresh parsley over the top, give everything a final gentle toss, and serve the beef and noodles with mushrooms hot.