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1
Preheat oven to 400F.
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2
Tear bread into pieces and pulse to coarse crumbs in a food processor.
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3
Transfer to a bowl and toss with oil, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
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4
Spread in a shallow baking pan and bake in middle of oven until golden, 6 to 8 minutes, then transfer to a bowl.
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5
Pull stems from mushroom caps (to create space for stuffing) and finely chop stems.
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6
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.
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7
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.
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8
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.
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9
Cool vegetables slightly, then add to bread crumbs along with cheese and parsley and toss well.
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10
Turn mushroom caps over, then mound mushroom filling in mushroom caps, pressing gently (there will be some filling left over).
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11
Bake in middle of oven until mushrooms are tender and stuffing is golden brown, about 20 minutes.
