- 3 slices firm white sandwich bread
- 2 tablespoons olive oil
- 1/2 teaspoon salt
- 1/4 teaspoon black pepper
- 20 large white mushrooms (2 to 2 1/2 inches in diameter)
- 2 tablespoons unsalted butter
- 1 medium onion, finely chopped
- 1/4 cup finely chopped celery
- 2 cloves garlic, finely chopped
- 1/2 teaspoon dried oregano, crumbled
- 1 ounce finely grated Parmigiano-Reggiano
- 1/4 cup chopped fresh parsley
- Preheat oven to 400 degrees F.
- Tear bread into pieces and pulse to coarse crumbs in a food processor. Transfer to a bowl and toss with oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper. Spread in a shallow baking pan and bake in middle of oven until golden, 6 to 8 minutes, then transfer to a bowl.
- Pull stems from mushroom caps (to create space for stuffing) and finely chop stems. Put mushroom caps, stemmed sides down, in a lightly oiled large shallow baking pan and bake in middle of oven until mushrooms exude liquid, about 10 minutes, then remove from oven.
- While mushroom caps are baking, melt butter in a 12-inch heavy skillet over moderately high heat until foam subsides, then saute chopped stems, stirring, until golden, about 5 minutes. Add onion, celery, garlic, oregano, and remaining 1/4 teaspoon salt and 1/8 teaspoon pepper and saute, stirring occasionally, until onion is golden, about 5 minutes. Cool vegetables slightly, then add to bread crumbs along with cheese and parsley and toss well.
- Turn mushroom caps over, then mound mushroom filling in mushroom caps, pressing gently (there will be some filling left over). Bake in middle of oven until mushrooms are tender and stuffing is golden brown, about 20 minutes.