Preheat a large slow cooker. Place all ingredients in it and stir together, except the cornstarch sour cream and noodles. Cook on low for 8 hours or high for 6 hours, until beef is tender. In a heatproof measuring cup, whisk cornstarch with 2 Tbsp boiling hot water. Ladle one cup of the cooking liquid from the beef into the measuring cup and whisk to combine. Pour into a small saucepan and bring to a boil. Cook until thickened, about 1 minute. Turn off slow cooker and stir in the cornstarch mixture, then sour cream. Serve beef over cooked egg noodles.