In a large skillet, brown the beef stew meat with the chopped onion over medium-high heat. Season with garlic salt. Cook until the onion is translucent and the meat is browned on the outside, but not cooked through. This step helps keep the meat moist and tender.

In a separate bowl, whisk together the cream of mushroom soup, milk, Worcestershire sauce, and cream cheese until well combined and mostly smooth. Stir in the sliced mushrooms and sour cream.

Transfer the browned meat and onion mixture to a slow cooker. Pour the sauce mixture over the meat. Stir to combine.

Cover and cook on low for 4-6 hours, or until the beef is tender. Stir occasionally.

About 15 minutes before serving, cook the egg noodles according to package directions. Drain the noodles.

Once the noodles are cooked, stir them directly into the slow cooker with the beef stroganoff. Let sit for about 5 minutes to allow the noodles to absorb some of the sauce.

Serve hot.