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In a small bowl, whisk together the soy sauce, mustard, Worcestershire sauce, garlic and pepper.
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Place flank steak in a glass bowl or resealable plastic bag.
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Pour marinade over the steak, cover and refrigerate for at least 6 hours or up to 2 days, turning occasionally.
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Preheat broiler or grill.
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Remove steak from marinade. Dispose of marinade.
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Broil or grill steak about 6 inches from the heat for about 7 minutes per side.
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Remove from heat and let the steak sit about 10 minutes before slicing thinly on the diagonal.