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Combine the oil and rest of the ingredients in a food processor or blender.
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Pulse until the peppers and cilantro are smooth.
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Place marinade in a large ziploc bag.
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Add large flank steak and mix the marinade around to completely coat the steak.
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Marinade for at least 8 hours, but up to 24.
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Grill large steak for 5-10 minutes per side (5 minutes for rare meat, 10 for well done).
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While steak is grilling, add marinade to a small saucepan.
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Bring to a boil and let simmer for 8-10 minutes.
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Cut the steak across the grain into thin diagonal slices.
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Serve steak with the marinade sauce.