Bring a medium pot of salted water to a boil and cook the egg noodles according to package directions until al dente.
While the noodles cook, slice the sirloin steak into thin strips and season with half of the sea salt and black pepper.
Heat the olive oil in a large skillet over medium-high heat and sear the beef strips until browned, about 2 minutes per side, then remove from the pan and set aside.
In the same skillet, add the sliced mushrooms and diced onions, sautéing until the mushrooms are browned and the onions are translucent.
Stir in 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 any browned bits, and simmer for 3 minutes to reduce slightly.
Lower the heat to low, then stir in the Greek yogurt and the remaining salt and pepper until the sauce is smooth and creamy.
Return the beef and any accumulated juices to the skillet, tossing to coat in the sauce.
Drain the egg noodles and serve the beef stroganoff over the noodles, garnished with freshly chopped parsley.