How to Make It
Combine mayonnaise, scallions, dill, yogurt, 1/4 cup parsley, lemon juice, water, anchovy paste, 3/8 teaspoon salt, and garlic in the bowl of a mini food processor; process until smooth, about 1 minute. Set aside.
Heat oil in a large cast-iron skillet over high. Sprinkle pepper and remaining 1/4 teaspoon salt over steak. Cook steak until well browned on bottom, about 5 minutes; turn steak, and cook to desired degree of doneness, about 5 minutes for medium. Transfer to a cutting board, and let rest 5 minutes before thinly slicing.
Meanwhile, toss together romaine, cucumber, radishes, and remaining 1/4 cup parsley in a large bowl. Divide among 4 plates. Top with sliced steak; drizzle with dressing.