Heat olive oil in a large skillet over medium-high heat.
Add the sliced beef and season with salt and pepper. Cook until browned, about 3-4 minutes. Remove the beef from the skillet and set aside.
In the same skillet, add the sliced onion and mushrooms. Sauté for about 5 minutes, or until the onions are translucent and the mushrooms are tender.
Return the beef to the skillet. Stir in the Dijon mustard, soy sauce, beef broth, and garlic powder.
Bring to a simmer and cook for an additional 5-7 minutes, allowing the sauce to thicken slightly.
Taste and adjust seasoning if necessary. Garnish with fresh parsley before serving.