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1
Preheat the oven to 375.
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2
In a very large skillet, melt the stick of butter.
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3
Add the shallots and cook over moderate heat until softened, about 4 minutes.
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4
Add the shiitake and white mushrooms, season with salt and pepper and cook over moderately high heat, stirring occasionally, until the liquid evaporates and the mushrooms are lightly browned, about 8 minutes.
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5
Add the sherry and sherry vinegar and cook for 1 minute.
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6
Season with salt and pepper.
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7
Transfer the mushrooms to a food processor and process until finely chopped but still a bit chunky.
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8
Lay 1 sheet of phyllo on a work surface and brush with melted butter, then top and repeat with 2 more phyllo sheets; keep the rest covered with plastic wrap.
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9
Halve the layered phyllo to form two 8 1/2-by-12-inch rectangles.
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10
Spoon 1/2 cup of the mushrooms at the bottom of each half, leaving a 1-inch border.
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11
Roll the phyllo over the mushrooms; fold in the sides and continue to roll up like a cigar.
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12
Brush each strudel with melted butter and transfer to a large baking sheet.
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13
Repeat with the remaining phyllo, mushrooms and butter.
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14
Bake the strudels for 15 minutes, or until nicely browned and crisp.
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15
Let rest on the baking sheet for 3 minutes.
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16
With a serrated knife, cut the strudels on the bias into 2-inch lengths.
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17
Transfer to a platter, cut sides up, and top with small dollops of sour cream.
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18
Serve hot.
