Preheat a skillet to medium or medium-low heat (depending on your stove top).
Cut 2 thin slices of sourdough bread.
Shred the cheeses.
Spread a small amount of the butter onto one of the slices of bread.
Place the remaining butter into the preheated pan and, as it is melting, place the unbuttered bread slice on top of the melting butter.
Place all of the cheese evenly over top of the bread slice in the pan. Then top with the remaining bread slice, butter side out.
Cover slightly with a lid and cook about 2-3 mins. Check to make sure it is not getting too dark.
After it is golden brown, and the cheese is melting, flip the sandwich over. Continue to cook on the other side until golden. (About 2 mins)
Remove from the pan, let cool slightly and serve warm.