Trim 6 of the scallions; halve lengthwise. Thinly slice remaining 2 scallions, and set aside.

Heat 2 tablespoons of the oil in a large cast-iron skillet over high. Add scallion halves, and cook, stirring often, until charred, about 4 minutes. Transfer scallions to a plate. Reduce heat to medium high.

Add remaining 2 tablespoons oil to skillet. Season both sides of steak with 1 teaspoon of the salt and ½ teaspoon of the pepper. Carefully add steak to hot oil in skillet. Cook, turning once, until a thermometer inserted into thickest portion of meat registers 125° to 130° for medium-rare, 8 to 13 minutes, or until desired degree of doneness. Transfer steak to a cutting board, and let rest 10 minutes.

Finely chop charred scallions. Place in a medium bowl, and stir in softened butter, lime zest and juice, garlic, red pepper and remaining ¼ teaspoon each salt and pepper

Slice steak diagonally against the grain; top with scallion-butter mixture and reserved sliced scallions. Serve immediately.