Cut the beef chuck roast into 1-inch cubes and set aside.

Heat 2 tablespoons of oil in a skillet, add beef cubes in batches, and sear until browned on all sides.

Sauté chopped onion and minced garlic in the same skillet until translucent, then add sliced mushrooms and cook until softened.

Combine seared beef and sautéed vegetables in the slow cooker with beef broth, Worcestershire sauce, and Dijon mustard. Stir well.

Cover and cook on low for 7-8 hours or high for 4-5 hours until tender.

Mix sour cream and flour until smooth, then stir into the slow cooker 30 minutes before serving.

Ladle over cooked egg noodles or rice and garnish with fresh parsley.