Place the onions and mushrooms in the bottom of a 6-quart slow cooker, then top with the cubed beef.
In a medium bowl, whisk together 2½ cups of the beef stock, the red wine, soy sauce, Worcestershire sauce, salt, pepper and garlic powder. Pour over the beef, place the lid on the slow cooker and cook on low for 6 hours.
Place the flour in a small bowl and gradually whisk in the remaining 1½ cups of beef stock. Stir into the beef mixture, place the lid back on and cook on high for an additional hour.
When ready to serve, cook the egg noodles according to the package directions. Drain, return to the pot, and toss with 2 tablespoons of salted butter.