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1
Soak porcini in boiling-hot water in a small bowl until softened, about 10 minutes. Lift out porcini and squeeze excess liquid back into bowl. Rinse porcini well to remove any grit. Strain soaking liquid through a sieve lined with a dampened paper towel into another bowl and reserve.
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2
Pulse porcini and white mushrooms in a food processor until finely chopped (but not minced).
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3
Cook shallots in butter in a large heavy skillet over medium heat, stirring, until beginning to soften, about 1 minute. Add mushrooms, reserved soaking liquid, wine, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring occasionally, until liquid has evaporated, about 8 minutes. Stir in parsley, then spread filling on a plate and chill, uncovered, until cold, about 10 minutes.
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4
Preheat oven to 425u00b0F with rack in middle.
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5
While filling chills, put stack of phyllo sheets on a work surface and cut in half crosswise, then stack halves and cover with plastic wrap. Remove 1 phyllo sheet from stack (keep remaining sheets covered) and arrange with a long side nearest you, then lightly brush with some duck fat. Top with another phyllo sheet and brush with fat. Spread about one fourth of mushroom filling in a narrow strip along edge nearest you to within 1/4 inch of each end, then roll up phyllo tightly around filling, leaving ends open. Transfer roll, seam side down, to a baking sheet and make 3 more rolls in same manner, transferring to baking sheet. Bake until golden brown, 12 to 14 minutes. Cool slightly on baking sheet on a rack, then brush length of each roll with a very thin line of truffle oil (if using) with tip of a cotton swab. Gently cut each roll crosswise into 8 pieces with serrated knife. Serve strudel warm or at room temperature.
