Point it at the location you want the plate of sandwiches to appear
A large plate of sandwiches that keeps refilling itself appears
This spell was used by Professor Minerva McGonagall in 1992 so that Harry Potter and Ronald Weasley could have their dinner in the Potions Class in the Dungeons without interrupting the Start-of-Term Feast in the Great Hall.
Behind the scenes
While the spell seems to be a form of conjuration, this is not possible since conjuration of food is not allowed according to Gamp's Law of Elemental Transfiguration. However, a Gemino Curse or related spell may somehow be involved, which causes the sandwiches on the plate to multiply, since multiplication of food is not a violation of Gamp's Law.