We're referring both to the ziti that the Swede and Merry eat at Vincent's in New York City and the ziti that Merry prepares for Dawn every Friday night. This counts as both "symbol" and "imagery." This delicious ziti presents a vivid picture which works on our senses (and growling stomachs), but more importantly it functions as a symbol of sadness and loss.
The idea of the Swede continuing to eat baked ziti at Vincent's for years and years after the tragic events of Merry suggest a deep love for her and a sort of communion with her through a shared love of a certain food.
The ziti is also part of Zuckerman's artist's palette. It's what he sees the Swede eating before he knows the Swede has a daughter who is a suspected murderer—he's observing the Swede so closely that he takes note of the fact that ziti is the Swede's usual order. He uses this detail to lend a sense of reality to the novel and to connect the "before and after" sections.