Heat olive oil in a large skillet over medium-high heat. Sear the beef strips quickly until browned, then remove and set aside.
In the same skillet, sauté onions until soft. Add garlic and mushrooms, cooking until mushrooms are tender.
Stir in Dijon mustard and beef broth, scraping up any browned bits from the pan. Let simmer for 5 minutes.
Lower the heat and stir in sour cream. Mix well until smooth, then add the beef back to the skillet.
Cook egg noodles according to package instructions. Drain and toss with butter.
Serve stroganoff over buttered egg noodles, garnishing with fresh parsley.