In a small bowl combine the flour and beef broth slowly, and combine with a whisk to remove any lumps.
Add the flour and beef broth to the slow cooker, along with the beef, onions, mushrooms, dried parsley, mustard, salt, pepper, and garlic and stir to combine.
Cook on high for the first hour, then switch to low for 7 hours, or until the beef is tender.
When it's finished, turn off the slow cooker, let stand 10 min. Temper the sour cream by adding some of the hot beef stroganoff liquid into the sour cream, before adding that mixture into the slow cooker to help prevent the sour cream from curdling. Finish by adding the fresh parsley and serve over noodles, rice or potatoes.