3 tablespoons unsalted butter, at room temperature
4 ounces shiitake mushrooms, stemmed and caps thinly sliced
1 scallion, thinly sliced
2 cups baby spinach (2 ounces)
Salt and freshly ground pepper
Eight 1/2-inch-thick slices of challah bread
4 ounces thinly sliced Gruyère cheese
Mustard and pickles, for serving
In a medium skillet, melt 1 tablespoon of the butter. Add the mushrooms and scallion and cook over high heat, stirring, until softened and lightly browned, about 7 minutes. Add the spinach, season with salt and pepper and cook just until wilted, about 1 minute.
Top 4 slices of challah with half of the cheese. Spoon the hot mushroom-spinach mixture on top and cover with the remaining cheese. Close the sandwiches and spread the outsides with the remaining 2 tablespoons of the butter.
Heat a griddle. Add the sandwiches and cook over moderate heat, turning once, until the cheese is melted and the bread is toasted, about 6 minutes. Cut in half and serve right away with mustard and pickles.