Add the beef pieces, flour, and salt to a gallon-size food storage bag. Close the bag and shake until the beef is well-coated. (You can also do this in a large bow, tossing the meat to coat it.)
Place a deep-sided skillet over medium-high heat. Add 1 tablespoon of olive oil to the pan and once hot, add some of the beef pieces. (You'll need to work in batches the the pan doesn't get overcrowded.) Sear the beef on all sides for 3-4 minutes. Once browned, place the beef into the slow cooker. Add another tablespoon of oil to the skillet and repeat.
Turn the heat down to medium and add another tablespoon of oil to the skillet. Add the onions and cook for 3-5 minutes to soften. Add the garlic and cook for another minute, stirring constantly.
Add the mustard, Worcestershire sauce, beef broth, and mushrooms to the skillet. Bring to a boil and scrape up any browned bits on the bottom of the pan. Pour everything in the skillet over the beef in the slow cooker.
Cover and cook on LOW for 6 hours. (see note)
Make a cornstarch slurry by whisk the cornstarch and water together in a small bowl. Pour the slurry into the slow cooker and stir slowly. Once the sauce reaches your desired thickness, turn off the heat. (The sauce will continue to thicken as it cools.)
Stir together the sour cream and heavy in a small bowl. Then pour it into the slow cooker and gently stir to combine.
Spoon the beef stroganoff over cooked egg noodles with a sprinkle of chopped parsley on top. Serve warm.
Notes
Nutritional information is for the beef and sauce only.
The beef can be cooked for 6-8 hours on LOW. If you go the full 8 hours, the beef will be fall-apart tender when you stir in the cornstarch.