"14th and 15th century western European pictures of table settings often show flat rectangular objects somewhat larger than a hand (the rest of the tableware is almost always round) [e.g. Luttrell Psalter ~1340, 1432 Flanders illustrated Decameron (BnF Arsenal ms 5070)] The material used is usually unclear but in some instances seems have a rim and to be wooden (rather than metal or bread - 1430s Alexander the Great manuscript see http://www.peartree12.freeserve.co.uk/gallery/manuscript2.html). The item is often shown being used as a personal cutting board. Probably they were cheap items that allowed diners to cut food without scratching expensive metal plate.
For practical reasons these trenchers are made of North American maple (perhaps European ash would have been ideal). Details are conjectural they are on the large end of the range illustrated, and more bowl-like than most, to make them more useful as picnic plates. From a rectangular blank they were worked with gouges/chisels and plane. As cheap, ablative items, they have been left unadorned. They are finished with pure tung oil and are food-safe and (with care) cleanable."