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combine maple syrup, soy sauce, vinegar, garlic, and pepper in a bowl.
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place flank steak in a plastic bag with marinade.
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refrigerate for 2 hours or overnight.
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strain marinade into a sauce pan and cook on low heat.
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meanwhile, heat the grill to medium-high.
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place steak on grill and cook 5 minutes per side.
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cut into thin slices against the grain.
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serve with the heated marinade.