Sprinkle flank steak with salt and a generous amount of freshly ground black pepper. Heat a wide heavy skillet. Do not add fat. When hot, cook seasoned steak until seared and well browned on each side (about 1 minute per side). Reduce heat and add 2 T butter. Cook 3 to 5 minutes on each side. For best results, the meat should be quite rare. Remove the meat from the pan and keep warm. Pour off the fat in the skillet and add the scallions and red wine. Bring to a boil and whisk in the garlic puree. Boil until the wine is reduced by half, thickened, and syrupy. As it boils, scrape up the browned bits in the skillet with a wooden spoon. Stir in the meat juices that have accumulated unter the flank steak. Boil for 1 or 2 seconds more. Remove from the heat. Gently swirl in the 2 T soften butter so that it incorporates into the wine. Quickly slice the meat, against the grain, into thin strips. Arrange the slices on a hot platter, pour sauce down the center of them and serve at once.