Bring a pot of salted water to a boil and cook the whole wheat egg noodles according to package directions, then drain and set aside.
Slice the sirloin steak into thin strips and season with 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.
In the same skillet, add the sliced mushrooms and diced onions, saut éing until the mushrooms are golden 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 the flavorful browned bits.
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 with the sauce and mushrooms until heated through.
Serve the beef and mushroom mixture over the cooked noodles and garnish with freshly chopped parsley.