Caramelized onions make a savory dressing for this steak salad. Tomatoes, capers, and radishes add color and flavor to the rest of this main dish.
Cut off bottom core of romaine. Rinse leaves in cold water; pat dry. Cut out heavy center vein. Tear leaves into bite-size pieces. Wrap lettuce in a dry kitchen towel or paper towels and store in the refrigerator for at least 30 minutes or up to several hours to crisp romaine.
Prepare the Caramelized Onion Vinaigrette as directed.
Divide the romaine among 4 individual dinner plates. Arrange 1/4 of the tomatoes, the blue cheese, the radishes, parsley, and the capers on each serving plate of greens. Set aside.
Slash fat edges of steak at 1-inch intervals, being careful not to cut into the meat. Place the steak on the unheated rack of a broiler pan. Broil the meat 3 inches from the heat for 6 minutes. Turn the steak. Broil the meat to desired doneness. Allow 6 to 8 minutes more for medium or 8 to 10 minutes more for well done. Insert an instant-read meat thermometer at an angle into center of steak (temperature should register 155 degree F for medium or 165 degree F for well done).
Remove meat from the pan. Cover cooked steak with foil and let stand for 5 minutes before cutting into bite-size strips. (The internal temperature of the meat will rise 5 degree F during standing time.) Arrange steak strips on top of the salad. Drizzle about 2 tablespoons of the Caramelized Onion Vinaigrette over each salad.
In a medium skillet, melt butter over medium heat. Add finely chopped onion. Reduce heat to medium-low and cook for 15 to 20 minutes or until the onion is very soft and golden, stirring occasionally. Sprinkle sugar over the onion and cook, stirring frequently, for 7 to 10 minutes more or until the onion starts to brown. Remove from the heat. Cool the cooked onion slightly.
In a screw-top jar, combine the caramelized onion with extra-virgin olive oil, balsamic vinegar, red wine vinegar, lemon juice, soy sauce, salt, and pepper. Cover mixture and shake well. Serve dressing immediately or cover it and store in the refrigerator up to 1 week. If chilled, let stand at room temperature 30 minutes before using. Shake before serving.