Season the sirloin strips evenly with sea salt and black pepper.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Sear the beef strips for 1-2 minutes per side until a golden crust forms, then remove from the pan and set aside.
In the same skillet, add the sliced mushrooms and diced onions, sautéing until the mushrooms release their moisture and turn golden brown.
Stir in the minced garlic and cook for 30 seconds until fragrant.
Deglaze the skillet with the beef broth, using a wooden spoon to scrape up all the flavorful browned bits from the bottom.
Whisk in the Dijon mustard and simmer the liquid for 2-3 minutes to allow it to reduce slightly.
Reduce the heat to low and gently fold in the Greek yogurt until a smooth, creamy sauce is achieved.
Return the seared beef and any accumulated juices to the skillet, tossing thoroughly to coat in the sauce.
Serve the creamy beef and mushroom mixture over the cooked whole wheat egg noodles and garnish with freshly chopped parsley.