Ingredients

• 1 1/4 sticks (10 tablespoons) unsalted butter
• 1 pound cremini mushrooms, trimmed and chopped
• 1/2 teaspoon salt
• 1/4 teaspoon black pepper
• 1/3 cup dry white wine
• 1/2 pound chilled Italian Fontina, rind discarded and cheese coarsely grated (2 cups)
• 2 tablespoons finely chopped fresh flat-leaf parsley
• 16 very thin slices firm white sandwich bread
• About 1/2 teaspoon white-truffle oil (optional)

Instructions

1. Heat 2 tablespoons butter in a 12-inch heavy nonstick skillet over moderately high heat until foam subsides, then cook mushrooms with salt and pepper, stirring occasionally, until liquid mushrooms give off is evaporated, about 8 minutes. Add wine and boil, stirring occasionally, until liquid is evaporated, about 5 minutes. Cool mushrooms to room temperature, about 10 minutes.
2. Toss mushrooms with cheese and parsley in a bowl.
3. Divide cheese mixture among 8 slices of bread (a scant 1/2 cup per slice), spreading evenly, then top with remaining 8 slices.
4. Heat 1 tablespoon butter in cleaned skillet over moderate heat until foam subsides, then cook 2 sandwiches, without turning over, until undersides are browned, about 3 minutes. Transfer sandwiches to a cutting board. Heat 1 tablespoon butter in skillet until foam subsides, then return sandwiches, browned sides up, to skillet and cook until undersides are browned, about 3 minutes more, transferring back to cutting board as cooked. Make 3 more batches in same manner.
5. Trim off crusts and cut each sandwich into 4 triangles. Top each triangle with a drop of truffle oil (if using).