To make the chimichurri sauce, mix the vinegar, water, garlic, 1 teaspoon salt, 1 teaspoon black pepper, and pepper flakes. Whisk in the oil. When everything's blended, whisk in the parsley. Heat a grill or stovetop grill pan until hot. Season the steak with salt and pepper and place in on the hot grill. For medium-rare, cook it for 3 to 4 minutes on each side, or until an instant-read thermometer inserted into the thickest part reads 140°F Trim the roots from the scallions and add the entire bunch to the grill just after you've flipped the steak. Cook the scallions until they're lightly charred, 4 to 5 minutes. Drizzle the steak with the chimichurri and serve with the grilled scallions.