Several cucumber slices in and around the sandwiches
A cucumber was a vegetable native to Earth used as food. When sliced, it was the main ingredient in cucumber sandwiches. (VOY: "Persistence of Vision")
Sub-commander T'Pol had a salad including cucumber in the mess hall in June 2151 during a conversation with Doctor Phlox about IDIC. (ENT: "The Andorian Incident")
Something similar in appearance to cucumbers grew on vines on Hanon IV. In 2373, the crew of USS Voyager was able to use the foodstuff as sustenance when stranded on the planet. (VOY: "Basics, Part II")
Placing cucumber slices over the eyes had health benefits. In 2381, the USS Cerritos engineering staff had their eyes covered with cucumber slices while receiving mani-pedis on the Dove. However, it was found that they were also using the slices to fool the stress sensors in their wristbands, taking advantage of cucumbers' hydrophilic skin. (LD: "Room for Growth")