Bring a medium pot of salted water to a boil and cook the egg noodles according to package directions until al dente.
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. Add the beef strips in a single layer and sear for 1-2 minutes per side until browned. Remove the beef from the pan and set aside.
In the same skillet, add the diced onion and sliced mushrooms. Sauté for 5-7 minutes until the mushrooms are golden 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. Simmer for 2 minutes to slightly reduce.
Reduce the heat to low. Stir in the Greek yogurt and the remaining salt and pepper until a smooth, creamy sauce forms (do not let it boil or the yogurt may curdle).
Add the seared beef back into the skillet and toss to coat in the sauce.
Drain the noodles and divide them into bowls. Top with the beef stroganoff mixture and garnish with freshly chopped parsley.