Assemble the sandwich with two slices of bread, adding the chicken and spinach mixture, followed by grated cheese, and top with another slice of bread.
Spread butter on the outside of the sandwich and grill in the skillet for 3-4 minutes on each side until golden brown.
Remove from skillet, let cool for a minute, then slice in half and serve warm.
Notes
For variations, consider adding sun-dried tomatoes, different cheeses, or avocado slices. Serve with tomato soup or a side salad.