Place the beef stew meat, sliced mushrooms, diced onion, and minced garlic in the slow cooker.
In a bowl, whisk together beef broth, tomato paste, worcestershire sauce, dijon mustard, salt, black pepper, and dried thyme. Pour the mixture into the slow cooker and stir to combine.
Cover and cook on low for 7 to 8 hours or on high for 4 to 5 hours, until the beef is very tender.
In a small bowl, whisk the cornstarch with 3 tablespoons of cold water until smooth. Stir the slurry into the slow cooker.
Cover and cook on high for 15 to 20 minutes until the sauce has thickened.
Meanwhile, cook the egg noodles according to package directions. Drain well.
Turn the slow cooker to warm and stir in the sour cream until the sauce is smooth and creamy. Taste and adjust seasoning if needed.
Serve the stroganoff over the cooked egg noodles and garnish with fresh parsley.