Romaine lettuce, chopped up fine
cold cooked chicken breast, ditto
minced hard-boiled egg
crumbled Bleu cheese
Everything chopped up very small and mixed up with a Vinaigrette dressing (lemon juice, red wine vinegar, extra virgin olive oil, garlic, a dash of mustard powder and some fresh ground pepper).
This is adapted from the recipe given in the book Hollywood du Jour: Lost Recipes of Legendary Hollywood Haunts, by Betty Goodwin, which includes very interesting thumbnail histories of a lot of great vanished L.A. restaurants. I departed from the recipe by excluding the chicory (or Belgian endive), which I don't like and I don't recall being in the original salad served by the Brown Derby restaurants when I was a kid. Also, their recipe for Vinaigrette includes water and sugar and salt and is nonsense.
Here are some old pictures --click to enlarge-- which I scanned from a library book, whose name I've forgotten:
Carole Lombard and Clark Gable at the Vine Street Brown Derby, where he proposed to her. A caricature of William Powell -- Lombard's first husband, and frankly, a better actor than Gable -- peers over Gable's shoulder.
Then and now: on Vine Street looking Northeast at Hollywood Boulevard, the Brown Derby, right, with awning, has become a parking lot. I took the "new" photo in 2005.
Left: the Brown Derby at the corner of Wilshire Boulevard and Rodeo Drive, and it's later incarnation as a tacky Bulgari jewelers.