Heat the skillet: Heat a cast-iron skillet over medium heat.
Butter the bread: Spread butter evenly over one slice of sourdough bread, sprinkle lightly with salt, and place it buttered-side down in the skillet.
Add the cheese: Layer at least two slices of cheese on top, making sure they cover the bread edge to edge.
Butter the top slice: Spread butter on the second slice of sourdough bread, sprinkle lightly with salt, and place it buttered-side up on top of the cheese.
Cook the first side: Cook for 3–4 minutes, or until the bottom is golden brown and the cheese has begun to melt.
Flip and finish: Carefully flip the sandwich and cook for another 3–4 minutes, until the second side is golden and the cheese is fully melted.
Serve: Remove from the skillet, slice if desired, and serve warm with soup, bone broth, or on its own.