Boil water in a small pot and cook the soba noodles according to package directions, then drain and set aside.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Season the beef strips with half of the salt and pepper, then sear in the hot skillet until browned on all sides, about 4 minutes total.
Remove the beef from the skillet and set aside on a plate.
In the same skillet, add the sliced mushrooms and diced onions, sautéing until the onions are translucent and mushrooms have released their moisture.
Stir in the minced garlic and cook for 30 seconds until fragrant.
Pour in the beef broth to deglaze the pan, scraping up any browned bits, and whisk in the Dijon mustard.
Reduce the heat to low and stir in the Greek yogurt until the sauce is smooth and creamy.
Return the beef and any accumulated juices to the skillet, tossing to coat in the sauce until warmed through.
Serve the creamy beef and mushroom mixture over the cooked soba noodles and garnish with chopped fresh parsley.