Trim excess fat from the beef and cut it into 1- to 1 1/2-inch pieces. Season all over with the kosher salt, black pepper, and paprika.

For deeper flavor, brown the beef in a lightly oiled skillet in batches for 2–3 minutes per side. This step is optional, but it gives the finished stroganoff better color and a richer savory taste.

Place the chopped onion, sliced mushrooms, and minced garlic in the slow cooker, then add the seasoned beef. Spread the ingredients into an even layer so the cooking liquid can surround the meat.

Whisk together the condensed cream of mushroom soup, beef broth, Worcestershire sauce, and Dijon mustard until smooth. Pour this mixture over the beef and vegetables.

Cover and cook until the beef is very tender, 6–8 hours on LOW or 3–4 hours on HIGH. Avoid lifting the lid repeatedly because escaping heat can lengthen the cooking time.

Cook the wide egg noodles separately according to the package directions, usually for 8–10 minutes. Drain them well and keep them ready for serving.

Stir the sour cream into a small amount of the hot cooking liquid in a bowl until loosened and smooth. Return that mixture to the slow cooker and stir gently; do not boil the sauce after adding the sour cream.

Spoon the creamy beef stroganoff over the drained egg noodles. Finish with chopped parsley if desired and serve while hot.