Cook the soba noodles according to package directions, drain, and set aside.
Slice the sirloin steak into thin strips and season with half of the sea salt and black pepper.
Heat avocado oil in a large skillet over medium-high heat and sear the beef until browned, then remove from the pan and set aside.
In the same skillet, add the diced onions and sliced mushrooms, sautéing until the mushrooms are golden and the onions are translucent.
Add the minced garlic and cook for 30 seconds until fragrant.
Whisk together the beef bone broth and Dijon mustard, then pour into the skillet to deglaze, scraping up any browned bits.
Reduce the heat to low and stir in the Greek yogurt until smooth, ensuring the sauce does not boil.
Return the beef to the skillet and toss to coat in the creamy sauce, seasoning with the remaining salt and pepper.
Serve the beef and mushroom mixture over the cooked soba noodles and garnish with fresh chopped parsley.