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1
In a very small saucepan boil Sherry until reduced by about half and let cool.
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2
In a bowl whisk together Sherry, balsamic vinegar, garlic, shallot, sugar, and salt and pepper to taste and add oils in a stream, whisking until emulsified.
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3
Whisk in herbs and reserve 1/4 cup vinaigrette for dressing mesclun.
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4
Transfer remaining vinaigrette to a large sealable plastic bag.
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5
Discard stems from mushrooms.
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6
Add mushrooms to bag and seal, pressing out excess air.
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7
Put bag in a bowl.
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8
Marinate mushrooms, chilled, turning bag once or twice, at least 30 minutes and up to 2 hours.
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9
Preheat oven to 350F.
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10
Finely chop onion and cut prosciutto into julienne strips.
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11
In a skillet cook onion in butter over moderately low heat, stirring, until soft and transfer to a bowl.
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12
Stir in prosciutto and remaining stuffing ingredients and salt and pepper to taste until combined well.
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13
Remove mushrooms from bag, letting excess vinaigrette drip off, and arrange mushrooms, gill sides up, in a 13- by 9- by 2-inch baking pan.
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14
Divide stuffing among mushrooms and press gently and evenly into caps.
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15
Pour water and Sherry around mushrooms and bake in middle of oven 20 to 25 minutes, or until stuffing is golden brown.
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16
In a bowl toss mesclun with reserved vinaigrette and salt and pepper to taste and divide among 4 plates.
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17
Transfer mushrooms with a slotted spatula to a cutting board and cut each crosswise into 4 or 5 pieces.
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18
Transfer a sliced mushroom to each plate and garnish with Parmesan curls.
