Bring a small pot of water to a boil and cook the whole wheat egg noodles according to package directions until tender, then drain and set aside.
Heat the olive oil in a large skillet over medium-high heat.
Season the beef sirloin strips with sea salt and black pepper, then sear in the skillet until browned, about 2-3 minutes per side. Remove beef from the pan and set aside.
In the same skillet, add the sliced cremini mushrooms and diced yellow onion, sautéing until the mushrooms are golden and the onions are translucent.
Add the minced garlic and cook for 30 seconds until fragrant, then pour in the beef broth to deglaze the pan, scraping up any browned bits.
Reduce the heat to low. Stir in the Dijon mustard and nonfat Greek yogurt until the sauce is smooth and creamy, being careful not to let it boil.
Return the beef and the cooked noodles to the skillet, tossing gently to coat everything in the sauce.
Garnish with freshly chopped parsley and serve immediately.