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 are boiling, slice the sirloin steak into thin strips and season them 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, 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 onions are translucent and the mushrooms have released their moisture and turned golden.
Add the minced garlic to the skillet 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.
Reduce the heat 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 strips and any accumulated juices to the skillet, tossing to coat in the sauce until heated through.
Drain the egg noodles and divide them into a bowl, topping with the beef and mushroom mixture and a garnish of fresh parsley.