Melt butter in a medium skillet. Add onion and cook over medium heat about 25-30 minutes, stirring often, until onions are brown and caramelized. Use care not to let onions burn. Add sugar and cook until well incorporated, about 5 minutes. Season to taste with salt.
Place caramelized onions and remaining ingredients in a slow cooker. Heat, covered, on high heat until completely warmed through (about an hour), then reduce heat to low until ready to serve. Season to taste with salt and pepper.