In a large frying pan over low heat, whisk together peanut butter, soy sauce, brown sugar, hot sauce, lime juice and water to thin sauce to a pourable mixture. Set aside, keeping over low heat.
Season flank steak with salt and pepper. Grill or pan fry to medium rare 140°F. Once flank steak is cooked, let it rest 5 minutes and slice thinly across the grain. Place in the pan with peanut mixture to coat. Top with chopped peanuts and green onions.