Preheat your cast iron pan, oven, or griddle as you prepare your sandwich. Heat to 275°F. Butter both slices of the sourdough bread on each side of the bread.
If using mayo on your sandwich, add a thin layer of mayonnaise to one side of each slice of bread. The mayo side will be the outside layer of the sandwich.
Shred your cheese, if using a block of cheese. Otherwise, get your slices out and ready for your sandwich.
Once your griddle is preheated, lay your slices of bread, butter side down on the griddle. If using mayo, you want the mayo side of the bread to be the outside, so it will be toasted second.
Toast for 2-3 minutes, until golden brown. Flip your slices of bread.
Add the cheese to the toasted side of the bread while the second side toasts.
Place one piece of toast on top of the other piece of toast.
Slice on a diagonal and enjoy warm!