Chop cilantro finely and mince garlic. Combine in a bowl with cumin, pepper, salt, olive oil, brown sugar, lime zest, lime juice, and soy sauce to make the marinade.
Cut the sirloin flap steak against the grain into strips and coat with the marinade. Refrigerate for at least 1 hour, up to 4 hours.
Grill the marinated steak strips for a few minutes on each side until cooked to your desired doneness. Rest the meat before slicing.
Slice the cooked steak against the grain for tenderness. Serve in tacos, salads, quesadillas, or enjoy on its own.
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