It's a sandwich truck, and it's called Mama Cass. Get it? Well, apparently owner Cass Pham Thacher doesn't. She named the truck after herself, you see, and none of the press about it when it launched last summer latched on to the connection to the false but popular rumor that the real "Mama Cass" Elliot died from choking on a ham sandwich. Anyway, Thacher's Vietnamese sliders (three for $12!) already have a lot of fans, and you can catch the truck in Hayes Valley, at UN Plaza, and at Mint Plaza, by following the schedule posted here. Just don't tell the Cass Elliot fans.