Preheat a 10-12-inch non-stick skillet over medium-high heat for 2 minutes. Add the ground beef, allowing the pieces to sear upon contact. Using a wooden spatula or nonstick spoon to break up the beef into smaller pieces, cook the ground beef for 4 minutes until it is no longer pink. Drain the beef in a metal colander and set aside.
Reduce the heat to medium and add the olive oil to the 10-inch non-stick skillet. Add the chopped onion. Sauté for 2 minutes until the onion starts to soften, then add the garlic, salt, and pepper. Cook for an additional minute, then turn off the heat.
Add the drained cooked beef to the bottom of a 5 to 6 quart or larger slow cooker, then top with the seasoned onions & garlic. Add the white mushrooms and beef broth and mix until combined.
Add Worcestershire sauce and Dijon mustard, mix well using the wooden spatula. Cover and cook on high for about 90 minutes, or low for 2.5 hours, stirring occasionally.
Add the uncooked egg noodles to the slow cooker, and mix. Cover and cook for an additional 20 minutes, stirring every 5 minutes to ensure the uncooked noodles are fully covered and absorbing the liquid.
Stir in sour cream until combined, and allow to finish cooking for an additional 10 minutes, stirring occasionally.
Serve topped with sour cream and chopped fresh parsley, if desired.
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Notes
Leftovers: Refrigerate leftovers in an airtight container. Leftovers can be enjoyed for up to 3 days after cooking.
If you are feeding a larger crowd, double the recipe and use a 6-8 qt large size slow cooker to accommodate the ingredients, especially the pasta and liquid.