Cut the beef chuck into bite-sized pieces and place it into the slow cooker along with the sliced mushrooms and chopped onions.
Add the condensed mushroom soup, soy sauce, Dijon mustard, Worcestershire sauce, minced garlic, salt, and pepper, then stir everything gently until well combined.
Cover the slow cooker and cook on low heat until the beef becomes tender and the flavors have fully developed.
In a small bowl, mix the cornstarch and water until smooth, then stir it into the slow cooker to help thicken the sauce.
Let the mixture cook a little longer until the sauce reaches a slightly thicker consistency.
Turn off the heat and stir in the sour cream until the sauce becomes smooth and creamy.
Serve the stroganoff over hot cooked egg noodles and garnish with fresh parsley if desired.