- 1 stick (8 tablespoons) unsalted butter
- 2 pounds white button mushrooms, stem ends trimmed (not cut off) and halved if very large
- 2 celery stalks, cut into 1 inch pieces
- 2 large shallots, minced (about 1/2 cup)
- 3/4 teaspoon salt
- 1/2 teaspoon black pepper
- 1/3 cup medium dry sherry or port
- 1/2 cup seasoned breadcrumbs
- 1/3 cup finely chopped flat-leaf parsley
- 1/4 cup finely grated Parmegiano-Reggiano cheese
- 10 slices firm white sandwich bread
Melt butter over medium heat in a large (12-inch) skillet. While butter melts, pulse mushrooms in a food processor in 2 to 3 batches, until very finely chopped, transferring each batch as chopped into skillet with butter. Pulse celery in food processor until very finely chopped and add to skillet with mushrooms, then add shallots, 3/4 teaspoon salt and 1/2 teaspoon black pepper. Cook over medium-high heat, stirring occasionally, until liquid mushrooms release is evaporated and mushrooms are browned and dry, 20 to 25 minutes. Add sherry and cook until evaporated, 1 to 2 minutes. Transfer mixture to a bowl and cool to warm.
Once mixture is cooled, stir in breadcrumbs, parsley, and cheese. Season to taste with salt and black pepper.
Trim crusts from bread. Arrange slices on a large baking sheet. Lightly toast bread under broiler, 3 to 4 inches from heat, turning once, 45 seconds to 1 minute per side. Remove from oven. Spread 1/4 cup mushroom mixture evenly over each toast. Return to broiler until just a shade darker and warmed through, 1 1/2 to 2 minutes. Cut into quarters.
Make ahead: Mushroom mixture can be made 1 day ahead and kept chilled, covered.