Bring a pot of salted water to a boil and cook the egg noodles according to package directions until al dente, then drain and set aside.
While noodles cook, slice the sirloin steak into thin strips and season with sea salt and black pepper.
Heat olive oil in a large skillet over medium-high heat; sear the beef strips quickly until browned, then remove from the pan and set aside.
In the same skillet, add the diced onion and sliced mushrooms, sautéing until the mushrooms are browned and the onions are translucent.
Add the minced garlic and cook for 30 seconds until fragrant.
Pour in the beef broth and Dijon mustard, scraping the bottom of the pan to release the flavorful browned bits.
Reduce the heat to low and whisk in the Greek yogurt until the sauce is smooth and creamy.
Return the beef to the skillet and toss with the sauce to warm through.
Serve the beef and mushroom mixture over the cooked egg noodles and garnish with freshly chopped parsley.