Fourth ingredient in a classic three-ingredient sandwich (4)

I believe the answer is:

mayo

(Other definitions for mayo that I've seen before include "County of north-west Ireland" , "Salad dressing (abbr)" , "Ir. county; salad dressing" , "Salad dressing (informal)" , "Saucy thing" .)

I've seen this clue in The New York Times.

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