Place kraut in slow cooker.
Add corned beef brisket and the spice packet that came with it.
Add lid and cook on low for 12 until the corned beef is about to fall apart.
Remove the corned beef and chop. Yes, you can slice it if you prefer. Return the beef to the slow cooker and stir to combine.
At this point you can add the Thousand Island to the slow cooker and stir, if you desire, or simply slather it onto the bread slices.
Spoon kraut and beef mixture onto the bread. Note: I like to transfer the mixture to a strainer first. I press down on it lightly to remove as much moisture as I can. This prevents me from having a soggy (bleh!) Reuben.
Add cheese to sandwich and serve.