Cook the egg noodles in a pot of boiling salted water according to package directions, then drain and set aside.
Season the beef sirloin strips 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 until browned on all sides, then remove from the pan and set aside.
In the same skillet, add the diced onion and sliced mushrooms, sautéing until the mushrooms are tender 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 any browned bits, and simmer for 3-4 minutes to reduce slightly.
Reduce the heat to low and stir in the Greek yogurt until the sauce is smooth and creamy.
Return the beef to the skillet and toss to coat in the sauce, seasoning with the remaining salt and pepper.
Serve the beef and mushroom mixture over the cooked egg noodles and garnish with fresh chopped parsley.