Add the onions, butter, brown sugar, beer and salt to a slow cooker. Cook on low for 8 to 10 hours or until soft and dark. Stir occasionally, if possible.
Use the onions immediately, or store in an airtight container in the refrigerator for up to a week.
Butter one side of each slice of bread. Add a few slices of cheese to the non-buttered side, top with a tablespoon or two of the caramelized onions, a few teaspoons of goat cheese, then more slices of cheddar. Top with another slice of bread, buttered side out.
Cook in a pan over medium high heat until cheese has melted and butter is golden brown. Slice and serve immediately.
Notes
Use a darker, malty beer. A brown ale, stout, porter or Belgian ale will work really well.