A BLT sandwich was a type of Earth food made with the three primary ingredients of bacon, lettuce, and tomato, served with mayonnaise, on two slices of bread.
The nickname "BLT" was given to B'Elanna Torres by Max Burke when the two were dating the late 2360s. The nickname, which was based on the sandwich name, was also Torres' initials.
When Tom Paris learned of this, he became slightly jealous of Torres and Max's relationship. To "rub it in" a little, Harry Kim ended the conversation by referring to Paris as "turkey platter." (VOY: "Equinox")