To thicken sauce with cornstarch: Cook noodles as normal. When the beef is thoroughly cooked, remove beef and onions from the slow cooker to a bowl leaving the sauce in the crock. In a small bowl, combine 1 tablespoon of cornstarch with 1/2 cup of cold water. Stir thoroughly until no lumps remain. Whisk the beef sauce in the slow cooker continually as you stir in the cornstarch liquid. This will begin to thicken. Now you can stir in the sour cream until completely combined. Return the beef and onions to the sauce and stir to coat. Serve over noodles as usual.