Pat steak cubes dry and season evenly with salt and pepper. Heat large sauté pan over medium-high heat; add oil to coat pan. Sear cubes in batches in a single layer, developing a good browned crust on a least two sides, about 2 minutes per side. Transfer cubes to slow cooker.

Add butter, shallots and mushrooms to pan, stirring and scraping the browned bits from the bottom of the pan. Cook 5-8 minutes until water from mushrooms evaporates. Stir in paprika, mustard and stock and transfer to slow cooker; cover and cook on low for 5 hours.

In a small mixing bowl, whisk together sour cream and flour. Stir into cooker; cover and simmer an additional 30 minutes. Cook noodles according to package directions and drain. Stir noodles into stroganoff and serve, garnishing with parsley.