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In a large plastic bag, combine vingegar, soy sauce and sugar; add steak.
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Seal and refrigerate for 8 hours.
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Remove from marinade, reserving the marinade (used the reserved half if you doubled the marinade recipe).
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In skillet, heat oil over medium-high heat, cook onion, garlic, stirring, until transluscent, 3 minutes.
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Add mushrooms; cook until liquid is evaporated, about 10 minutes.
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Add reserved marinade; cook until no liquid remains, about 5 minutes.
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Meanwhile, broil steak, or grill over medium-high heat, for 7-9 minutes per side for medium rare.
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Transfer to cutting board. Tent with foil; let stand for 10 minutes.
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Slice thinly across the grain.
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Serve with mushroom mixture; sprinkle with parsley.