Measure 1/2 cup broth and reserve in covered container in the fridge. You will use this later, in Step 6.

Season beef on all sides with 1-1/2 teaspoons salt and 1/4 teaspoon pepper. In large skillet, heat oil over medium-high heat. Add half of beef and cook 6 to 8 minutes or until browned on all sides; turning occasionally. Transfer beef to bowl of slow cooker. Repeat with remaining beef, transferring beef to slow cooker when browned.

Add onion and mushrooms to skillet.  Cook 8 to 10 minutes or until mushrooms are browned, stirring occasionally. Stir in remaining 1/2 teaspoon salt and 1/4 teaspoon pepper, then transfer to bowl of slow cooker.

Add garlic, mustard, Worcestershire sauce and remaining broth from can, to bowl of slow cooker and stir well. Cover and cook on low for 8 hours, or on high for 4 hours.

When slow cooker timer is up, remove lid and use wooden spoon to coarsely break up beef. It’s ok if it’s still kind of chunky; you just want the pieces to be bite sized. 

In small bowl, whisk cornstarch and remaining 1/2 cup broth (from fridge) until smooth. Stir cornstarch mixture into beef mixture. Return the slow cooker to the HIGH setting, if necessary. Cover slow cooker and cook 15 to 20 minutes longer or until sauce has thickened.

Meanwhile, cook noodles according to package instructions and drain.

Add cream cheese and sour cream to slow cooker and stir until cream cheese is melted. Serve beef mixture over noodles, or pour cooked noodles into slow cooker and stir until well combined. Serve garnished with parsley, if desired.