Use 1 tbsp butter to butter the slices of bread with butter on one side.
Place half of the slices of bread in the pan, butter side down. Use half the cheese to top each slice of bread.
Layer the mushrooms on top and sprinkle the coriander, if using.
Layer on the remaining cheese.
Top each toastie with the remaining slices of bread, butter side up. Put a lid on the pan to help the heat to reach the cheese.
Flip the toasties when they’re golden underneath and replace the lid again.
When the bread is golden on both sides and the cheese is melted, remove from the pan, cut in half and serve.