Bring a medium pot of salted water to a boil and cook the whole wheat egg noodles according to package directions until al dente.
While the noodles cook, slice the flank steak against the grain into thin strips and season with the sea salt and black pepper.
Heat the ghee in a large skillet over medium-high heat and sear the steak strips for 1-2 minutes per side until browned, then remove from the pan and set aside.
In the same skillet, add the diced yellow onion and sliced cremini mushrooms, sautéing for 5-7 minutes until the mushrooms are golden and the onions are translucent.
Stir in the minced garlic, dijon mustard, and coconut aminos, cooking for an additional minute until fragrant.
In a small bowl, whisk the arrowroot powder into the beef bone broth until smooth, then pour the mixture into the skillet.
Simmer the sauce for 2-3 minutes until it begins to thicken, then reduce the heat to low.
Whisk in the plain Greek yogurt until the sauce is smooth and creamy, being careful not to let it boil.
Return the seared beef and any accumulated juices to the skillet, tossing to coat in the sauce.
Serve the beef and mushroom sauce over the cooked noodles and garnish with freshly chopped parsley.