Butter one side of the bread slices. Just butter the side that goes down on the pan.
Layer on the cheese. Add any optional ingredients.
Top the sandwich with the second piece of bread. Place in the skillet to pan-fry for 2 minutes, until cheese starts to melt. Flip and continue cooking for 2 more minutes, until golden brown.
Remove from heat and serve warm with a sprinkle of salt, side of veggies and additional protein.