3 slices firm white sandwich bread
2 tablespoons olive oil
1/2 teaspoon salt
1/4 teaspoon black pepper
20 large (2 to 2 1/2 inches in diameter) white mushrooms (1 lb)
2 tablespoons unsalted butter
1 medium onion, finely chopped (1 cup)
1/4 cup finely chopped celery
2 garlic cloves, finely chopped
1/2 teaspoon dried oregano, crumbled
1 oz finely grated Parmigiano-Reggiano
(1/2 cup)
1/4 cup chopped fresh parsley