Lettuce was an Earth vegetable generally eaten raw. It was the main ingredient in most salads.
The Third Doctor ate cheese sandwiches that Jane Blythe brought Jo Grant, leaving her with only the lettuce garnish. Jo was brought another platter to amend for the Doctor's rudeness. (TV: The Sea Devils)
Sarah Jane Smith insulted Kaleidoscope by calling him a "wet alien lettuce". (AUDIO: Kaleidoscope)
The Sixth Doctor's favourite sandwich consisted of peanut butter, lettuce and potato chips. (PROSE: Timeshare)