Add the lean beef stew meat, sliced mushrooms, chopped onion, and minced garlic to the slow cooker.
In a small bowl, whisk the flour into 1 cup of the beef broth until smooth. Pour this mixture into the slow cooker along with the remaining beef broth, Worcestershire sauce, and Dijon mustard. Stir to combine.
Season with salt and black pepper, then cover and cook on low for 7 to 8 hours or on high for 4 hours until the beef is tender.
About 20 minutes before serving, cook the whole wheat egg noodles according to package directions in a large pot of boiling water. Drain and set aside.
Stir the Greek yogurt into the slow cooker until the sauce is creamy and well blended. Let it warm through for 5 minutes.
Serve the stroganoff over the cooked noodles and top with chopped fresh parsley.