In English I can say "what about X?" to mean "why did you mention X?", "what were you going to say about X?", "what does it have to do with X?" and so on.
How can I ask the same thing in japanese? In particular, in casual speach.
Note that this is not what I'm asking about: "I like sushi. How about you?"
My question about this: for example, we're walking with my friend and suddenly she yells "sushi!". And then she says nothing. Strange. I don't get her and ask her "Sushi? What about sushi"?