How to Make It
In a large nonstick frying pan, heat 1 tablespoon of the butter over moderate heat. Add the mushrooms, salt, pepper, and dried sage, if using, and cook, stirring frequently, until golden brown, about 5 minutes. Stir in the fresh sage, if using. Put the mushrooms in a bowl and wipe out the pan.
Using a pastry brush, coat one side of 4 slices of the bread with half of the melted butter. Put them, buttered-side down, on a work surface. Top the bread with the cheese and then the mushrooms. Cover with the remaining 4 slices of bread; brush the tops with the remaining melted butter.
Heat the frying pan over moderately low heat. Add the sandwiches and cook, turning once, until golden, about 2 minutes per side.