Bring a pot of salted water to a boil and cook the egg noodles according to package instructions until al dente, then drain and set aside.
Thinly slice the flank steak against the grain into bite-sized strips and season evenly with sea salt and black pepper.
Heat the olive oil in a large skillet over medium-high heat and sear the beef strips until browned on both sides, then transfer them to a plate.
In the same skillet, add the sliced mushrooms and diced onions, sautéing for 5-6 minutes until the onions are translucent and the mushrooms have released their moisture.
Stir in the minced garlic and cook for 30 seconds until fragrant.
Pour in the beef broth and Dijon mustard, using a wooden spoon to scrape up any browned bits from the bottom of the pan, and simmer for 3 minutes to reduce slightly.
Turn the heat down to low and stir in the Greek yogurt until the sauce is smooth and creamy, being careful not to let it boil.
Return the beef and any accumulated juices to the skillet, tossing to coat in the sauce until warmed through.
Serve the beef stroganoff over the cooked egg noodles and garnish with chopped fresh parsley.