I'm no lawyer, but pretty sure witnesses don't come into play in a grand jury hearing. Just the prosecutor, explaining why it is worth the court's time to consider the case.
The phrase is a ham sandwich. That most grand juries will indict a ham sandwich if the prosecutor asks them to. Originally penned by Tom Wolfe in the book, Bonfire of the Vanities.
You're correct on ham sandwich. I'm old and used to carry one in a paper bag to school, so I guess my brain made that association.