Cut the sausage into smaller sections.
Add the butter to a large cast iron skillet and heat over medium heat until bubbling.
Add the sausage and cook, turning often, 5 to 7 minutes or until lightly browned. Transfer the sausage to a plate and set aside.
To the same skillet, add the diced onion and cook over medium heat, stirring often and scraping any browned bits from the bottom of the skillet. Cook for 5 to 7 minutes or until the onion is translucent.
Stir in the sauerkraut, brown sugar and caraway seeds (if using). Mix well.
Add the sausage back into the skillet, nestling the sections into the sauerkraut. Cook 10 to 15 more minutes or until the sausage is reheated.
Season to taste with pepper.
Serve with condiments such as stone ground mustard on the side.