Add chopped onions, chicken stock, and corned beef to the slow cooker. Set to low and cook for 8 hours.
To make gravy, remove 1 cup of liquid from bottom of slow cooker and all the onions. Put it into a medium size skillet and heat on medium. Whisk in flour to make a thick paste.
Slowly whisk in beer and brown sugar. Simmer for 15 minutes and until you get a thick gravy consistency.