Place potatoes in a large pot and cover with cold water. Bring the water to a boil and cook potatoes for 10 to 15 minutes until just cooked through. Transfer potatoes to a cutting board.
While the potatoes are cooking, combine parsley, cilantro, oregano, garlic, vinegar, oil, water, 1/2 teaspoon salt, 1/4 teaspoon black pepper and red pepper flakes in the bowl of a food processor. Pulse several times until the herbs are finely chopped but the mixture isn't a smooth paste. If you don't have a food processor, you can just finely chop all the herbs and whisk all the ingredients together in a medium bowl. Transfer to a small bowl.
Preheat an indoor grill pan over medium-high heat. Cut the potatoes in half lengthwise and brush with the herb mixture.
Season both sides of the flank steak with the remaining salt and pepper and place on the grill. Grill the first side for 4 minutes, turn and brush on the herb mixture. Continue to cook the steak for another 4 minutes, turn and brush the other side with the herb mixture. Continue to cook the steak for another 2 or 3 minutes, turning one more time, until the steak is cooked to desired doneness (total cooking time of 10 minutes for medium rare). Transfer steak to a cutting board and let rest for 10 minutes.
While the steak is resting, place the potatoes cut-side down on the grill and cook for 4 minutes. Turn and cook for another 2 or 3 minutes until cooked through.